I'm not going to be saying anything particularly original, here. Just a heads-up.
Probably the most important decision you'll make is whether or not you're going to run a game in an established setting.
The Forgotten Realms should be familiar to anyone who was around early enough to know they don't like Planescape. The Third Edition line of books have wonderful production values, and are
generally considered to be well-written and useful. The core books to get for a revised Third Edition game are the
Forgotten Realms Campaign Setting and
Player's Guide to Faerun hardcover books. You may also find
Monsters of Faerun useful; the other books in the line cover gods, villainous organisations, magic, and various regions -
Silver Marches,
Underdark, and so on.
Eberron is new, so you have the opportunity to get in on a new setting at the very beginning. As others have said, it was specifically designed to include everything found in revised Third Edition - which includes psionics as found in the
Expanded Psionics Handbook, though they are mostly confined to Sarlona, a separate continent from the main continent of Khorvaire. The other element of Eberron worth mentioning is that it tries to blend the prevalence of magic in D&D with a kind of pulp-adventure aesthetic; Khorvaire has just emerged from a century-long war
a la World War One, and to a large extent the society has the tone (if not the technology) of 1920s and 1930s pulp and noir fiction. The setting's being supported with a line of connected adventures - there's a short introductory adventure in the back of the
Eberron Campaign Setting hardback, which can lead into
Whispers of the Vampire's Blade,
Shadows of the Last War and
Grasp of the Emerald Claw.
Races of Eberron is due out soon, and
Monster Manual III includes information on how to fit many of its creatures into the setting, as well as the Forgotten Realms.
Those are the two settings currently supported by Wizards of the Coast; Greyhawk is pretty much confined to the RPGA Living Greyhawk campaign, which causes much wailing and gnashing of teeth from Greyhawk fans.
If you're interested in planar adventures, the two books you're looking for from Wizards are the
Manual of the Planes and the
Planar Handbook. The former is aimed at DMs (and much of its pre-revision information is included in the planar section of the
Dungeon Master's Guide), the latter at players. The latter also contains quite a bit of Planescape-derived material, though without the flavour of the setting, so it may not irritate you too much. Look over it in a store if you can to see if you find it useful.
There is a series of slim hardcover sourcebooks -
Complete Warrior,
Complete Divine,
Complete Arcane. and
Complete Adventurer, of which only the first two have so far been released - which aim to expand player choices along a number of obvious themes, as opposed to offering options for specific classes as the pre-revision "splatbooks" did; however, most people find
Complete Divine useful only for clerics, paladins, and druids, as opposed to
Complete Warrior which had material for all classes (though obviously more for fighter-types than, say, wizards).
Complete Adventurer seems like it will mostly be a catch-all skill-focused product most useful for rogues and the like. Each book presents three new 20-level base classes. Fair warning: if you don't want new feats and prestige classes, and a lot of them, you might not have much use for the
Complete X series.
Likewise, there is a series of products -
Races of Stone,
Races of the Wild, and
Races of Destiny - which provide more detail on the various standard races - dwarves and gnomes in
Races of Stone, elves and halflings in
Races of the Wild, and humans, half-elves, and half-orcs in
Races of Destiny. Each book also presents a new race - goliaths in
Races of Stone, illumians in
Races of Destiny, and an as-yet-unidentified race in
Races of the Wild. Like the
Complete X series, there's apparently quite a lot of feats, spells, and prestige classes in these books, but mixed in with what sounds like a decent dose of cultural information, too. There may be more products to come further on, as well; it's unclear as to how big the line is intended to be.
The
Book of Vile Darkness is a pre-revision but usable product covering "vile" paragons of Evil - theoretically, the darkest, most mature kinds of villains, including fiend-worshippers and the like. The
Book of Exalted Deeds is its post-revision counterpart, covering the champions of Good. Balance questions surround both these products, and the particular take on the material will not be to everyone's tastes, but I for one am happy to own them.
Now for the single-shot books . . .
The
Miniatures Handbook is a strange book, half of which is filled with base and prestige classes, feats, spells, and monsters usable in a normal D&D game, the other half of which contains a skirmish game system intended for use with the randomised, plastic, pre-painted collectible D&D Miniatures line.
The
Epic Level Handbook is a pre-revision sourcebook on playing characters past 20th level, and is not generally regarded as a well-balanced and playtested book. Most people agree the monsters are useful but scorn the epic-level spell seed system in particular. It's a pre-revision product, and some of its elements can be found in the epic section of the
Dungeon Master's Guide. It's also something which is supported to a pretty fair extent in the post-revision range - the
Complete X series and the
Expanded Psionics Handbook contain epic-level sections.
Speak of the Devil, the
Expanded Psionics Handbook, despite the name, is a standalone product for running psionics in revised Third Edition. By all accounts, despite some editing issues, the psionics system contained therein is the best implementation of psionics to be found in the whole history of D&D, and the author (Bruce Cordell) has also written what most consider to be the best third-party d20 sourcebooks for psionics. The
Expanded Psionics Handbook rules are seeing a bit of support in the
Races of X range, too, which is nice, as well as in Eberron.
Unearthed Arcana is a product containing variant rule systems. Alongside small changes like variant takes on the classes and additions to the system like Sanity (derived from d20
Call of Cthulhu), it contains more radical subsystems like spell points. Basically, it's a compilation of often mutually-exclusive ways in which you can do things differently. It's worth picking up if you're an inveterate rules-tweaker, or if more than one or two things about standard D&D grate on you, or if (like me) you're just crazy for new options.
Oriental Adventures is, as the name suggests, much like its previous editions' counterparts in that it helps you run a game based on Asian and other "oriental" legends and ideas. The default setting in that book is the
Legend of the Five Rings RPG and CCG setting, Rokugan, which has its own series of d20 sourcebooks published by AEG, starting with the
Rokugan Campaign Setting hardcover and continuing for about twenty volumes by now.
Oriental Adventures (and, incidentally, the entire AEG Rokugan line) is a pre-revision product, but
Dragon Magazine #318 has an article containing updates to bring it in line with the revision.
Ghostwalk is a campaign mini-setting, detailing a city and surrounds where death works differently, and the dead can manifest as non-undead ghosts and adventure alongside their friends until they can be raised or until they pass on to the next world. It's intended to work either as its own setting or as something you can drop into your own world. It's a pre-revision product, but there's a downloadable PDF update document on the Wizards of the Coast website.
I can also recommend a few d20 sourcebooks.
Monte Cook's
Arcana Unearthed, from his Malhavoc Press company (and
not to be confused with
Unearthed Arcana), is a "variant player's handbook". Basically, it's a different way to do D&D - it has its own races, its own base classes, its own magic system which makes no distinction between divine and arcane magic, and its own "implied setting" with its own assumptions which affect the system in the same way the assumptions of the pseudo-Greyhawk setting of core D&D affect that system. Even if you're happy with how D&D works, it's worth picking up for ideas.
Similarly, the
Complete Book of Eldritch Might from the same company is worth getting. Apart from a truckload of spells, it contains variant sorcerer and bard classes, prestige classes, magical sites, feats, and so on. It's basically a treasure-trove of arcane magical
stuff, compiling the three
Books of Eldritch Might Malhavoc had previously published and updating them to be compatible with revised Third Edition. It also has an appendix for converting everything, especially the spells, to
Arcana Unearthed.
The
Book of Hallowed Might and the
Book of Hallowed Might II are, obviously, divine magical sourcebooks, and the forthcoming
Book of Iron Might is a warrior-focused sourcebook. Monte Cook was one of the original designers of Third Edition before founding his own company, so he's in a relatively unique position when it comes to compatibility with standard D&D (though he tends to have a different design philosophy, especially compared to revised Third Edition in which he played no part, which is especially obvious in
Arcana Unearthed; I hasten to add, however, that what he does publish is perfectly compatible with the revision) compared to other d20 publishers, having been there on the ground floor as it were.
If you like a more traditional approach to the planes than Planescape offered, you might be interested in Malhavoc'
Anger of Angels, which attempts to represent Judaeo-Christian angelology in a D&D-compatible way. There's also, as you know,
Beyond Countless Doorways, which
isn't Planescape but
was designed by the Second Edition Planescape crew, so it may still have an approach you don't appreciate. However, it does lack Sigil, the cant,
et cetera, so if that's what bothered you about Planescape it may be perfectly suitable.
