Playing a level 24 wiz in our epic game here's what I'd recommend:
If you want an utterly absurd combination that is guarenteed to make your GM hate you take Arcana Form, Duplicate, and Prorogate Death, all from Monty's BoEM. (what was he thinking) Arcana Form is a L9 that makes you non-corporeal and allows you to burn HP @ 5 hp / spell level when casting spells; instead of losing the spell from your head you lose the hp. Combined with Prorogate Death (L6) which keeps you from dieing from negative HP (for at least a while) and duplicate (L9) which gives you a copy of yourself (now you get 2 hp pools and 2 sets of actions to burn your hp 2x as fast
). With this combination you will be a spellcasting machine and your gm will hate you.
Remember when using Arcana Form you will never cast any spell w/o burning hp to cast it, so you never have to choose the same spell twice when picking spells. I'd recommend you pick up a pearl of power 9 to recall the Arcana Form that you cast, just before it runs out you can rebuff using your hp to power additional castings.
Firebrand is useful as others have mentioned, Magma Burst from BoEM again is pretty neat (changes an area to lava, 20d6 / round while in the area), Mangle of Egregious Might (BoEM yet again), +4 LUCK bonus to stats/saves/AC/skill checks, Quintelemental Blast (BoEM, have you figured out yet Monty's stuff is over the top?), 15d6 damage, spell auto-picks the element that each target is least resistant to, Dragonsight (draconomicon, gives you absurd darkvision and blindsense distances).
The old (3.0) Simbul's sequencer spells are great (read too powerful) they have been totally nerfed in 3.5 and not worth it IMO, but if you can take the 3.0 ones have fun.
Lower Spell Resistance (dragononimcon) or the newer one whose name escapes me (in the planar handbook) are musts, will let you effectively ignore SR. Clerics can take these too, and it's useful to have more than one caster as you will have a giant bulls eye on your head when you start casting it if the target figures it out. (full round casting time)
Dimensional Anchor to nail down those pesky demons/devils/casters with teleport abilities.
Burning Bolt, I don't know what book it's in, I get it from Green Ronin's Pocket Grimoire Arcane. Like magic missile, but fire instead of force, each missle requires a ranged touch attack, but there is NO cap on the number of missles. Yes a level 1 spell will give your level 20 wiz 10 RTA's for 1d4+1 fire each.
There is a new Green Ronin book out whose name escapes me (about planetouched), I was just looking at a friend's copy this weekend, it has a bunch of spells in it. Look Bogg's XXX; there are four and three of the four are amazing (two do unnamed damage as I recall) and well worth taking.
Spells you will have that are very useful and should always be in your head:
Maze: The ultimate "go to your room" spell for fighting types.
Energy Drain: 2d4 negative levels hurts, and will lower saves making further magic easier
Mark of Death: another Monty spell (BoEM again) that will make you immune to death effects. High tech @L8 but lasts a long time
Magic Circle Against Evil: As written shuts off ALL enchantment/charm spells, although your GM may well only allow it vs. effects from evil casters since this is so powerful given it's level. If he does that take Mind blank and shut everything off anyway.
Bigby's grasping hand: Uses to-hit and grapple instead of save, making it a super reliable way to take out just about anybody (my L24 wiz has a +53 grapple check with this spell, can grapple giants easily).
Feats you will want:
Energy Sub (sonic), probably one more version for targets that start taking Energy Immunity. Acid is a good second choice.
Dragon Magazine ran some feats a while ago, about 18 months I think. I forget the exact issue. You want to find the Spellstrike feat, it allows you to cast spells where any time they do damage to a target the target has to make a DC(10 + caster level or 30 for you) balance check or fall prone. You need to use a 25gp gem as an additional component and the spell has a somatic component you cannot drop.
The sudden metamagic feats can be useful as you can apply them at casting time.
For magic items:
The biggest +INT item you can get
The biggest +CON item you can get (especially if you use Arcana Form)
A +5 INT book
As many rods of maximization as your GM will let you have
As many rods of quicken as your GM will let you have
A quiver of Ehlonna to hold all those metamagic rods, think of it as your golf bag
A few pearls of power to reset key spells
Vest of Restance +5
Stone of good luck (+1 luck bonus to saves is the biggie here)
Bracers of AC (if you have the arms and eqipment guide there is a blurb in there where the bracers can have armor effects on them, and can go up to +12 including AC and effects). Look through all the books for interesting effects, one I have is soulfire which makes you immune to death effects.
If you want to redo the character a bit taking 10 levels of either elemental savant or incantrix is a substantial boost over straight wizard. Elemental savant will give you some offensive abilities, but the real cookie is the immunities you get at level 10, immune to critical hits, flanking, poisons, stunning, and a few other things that escape me. Those immunities are very important at high levels. Incantrix will let you have all kinds of fun with metamagic, but you will need to invest in the metamagic feats.
Keep in mind you can use the various knowledge skills to figure out special abilities of monsters, always use this when fighting a monster you don't know. (and crank up those int skills) Use this to figure out their resitances to various magical effects and hit them at their weak points.