Probably best to avoid the early line of splattbooks:
Sword and Fist
Masters of the Wild
Tome and Blood
Song and Silence
Defenders of the Faith
The above are being replaced by a 3.5 compatible line beginning with the Complete Warrior and continuing with the Complete Divine in May and the Complete Arcane later, a presumably one more book.
Also, the Psionics Handbooks will be replaced by the Expanded Psionics Handbook in April.
Otherwise, you can buy most books with no particular problem; it just depends on your interests. WotC released a conversion document (the link is on the main D&D page) that updates the following: Dieties and Demigods, Epic Level Handbook, FIend Folio, Monster Manual II, and Manual of the Planes. In any event Fiend Folio is virtually 3.5e compatible as is.
One interesting question is Manual of the Planes. Will material in that book largely be replaced by the Player's Planar Handbook this summer?