Your must have choices are quite good. Since I don't know what is already in your collection, I would recommend the following:
*Spycraft (AEG) - best modern spy game since the original Top Secret.
*Call of Cthulhu d20 (Wotc) - The insanity rules are great and I really like the flavor and setting.
*Oathbound: Domains of the Forge (Bastion Press) - If you liked Planescape and Dark Sun, you'll love this. The prestige races are a cool concept.
*d20 Modern (Wotc) - Lots of flexibility with this one. Definitely has a greater story and role-play aspect to it than other Wotc products.
*Monsternomicon (Privateer Press) - Hands down, the best monster book on the market. The artwork is great IMO.
*Mercenaries (AEG) - Their best one-word title yet. Lots of really good classes and crunchables...
*Dragonstar: Starfarer's Handbook (Fantasy Flight Games) - Ties science fiction and fantasy together in an interesting fashion.
*Spycraft: Modern Arms Guide (AEG) - If you plan on running a modern campaign at all, this book belongs on your shelf.
*Book of the Righteous (Green Ronin) - Everything Deities & Demigods should have been in terms of pantheon creation and developing religions in your campaign world.
*Arms and Armor (Bastion Press) - An impressive array of armor and weapons, some mundane and most magical.
*Necropolis (Sword & Sorcery) - Egyptian flavored mega-campaign. Worth the money. Will keep your players busy for some time to come. Assuming they live.
*Path of the Sword/Path of Magic (Fantasy Flight Games) - Some really great fighter/mage based material. I rate this better than Mongoose's Quintessential series.
*Monstrous Handbook (Fantasy Flight Games) - Giving monsters classes and playing them smartly. This is the best treatment I've seen so far regarding this. If you don't want to wait until Wotc's Savage Species comes out sometime next year, pick this one up.
*Arcana: Societies of Magic (Green Ronin) - This is a hidden jewel. Filled with organizations you can drop into a campaign. The price is right for the quality that you get.
*Forgotten Realms Campaign Setting (Wotc) - Very richly detailed world. If you don't already own this, you should.
*Kingdoms of Kalamar Player's Guide (Kenzer) - Excellent player and DM resource. Great classes and spells. It should be part of your core rulebooks.
*Faiths and Pantheons (Wotc) - If you play in Forgotten Realms, add this to your collection. The rules for deity creation are the same as those in Deities and Demigods.
*City of the Spider Queen (Wotc) - This puts the word 'lethal' back in drow. It will make your players fear them forever more...
*The Banewarrens (Malhavoc) - A better written adventure than Return to Temple of Elemental Evil.
*Occult Lore (Atlas Games) - Adds some great magical flavor and optional classes to your campaigns. Very well written.
*Manual of the Planes (Wotc) - Another strong offering that broadens the possiblities and adventure ideas. Probably one of Wotc's best books to date. This have the updated githyanki and some excellent templates.
*Traps and Treachery (I and II) (Fantasy Flight Games) - A must have on any DM's shelf. These books give you a wide range of original traps that are guaranteed to leave your players cringing and more careful about simple things like coins laying on the floor.
There are others I could recommend but these are solid buys that you can use more than just once or twice through the years. Let us know what you finally wind up buying...
