Just make sure you need the heat.
The majority of the projects are indeed CPU only though. Milkyway and Einstein (and soon to be Universe) are best for AMD cards, especially those cards with high double precision (7970 and 280X). For Nvidia you have several math choices, and for medical research you have Folding of course, and GPUgrid. POEM is no more.
Seems like a short list, what have I missed fellas? Asteroids...actually one of the better CPU paying projects. SETI of course, although they seem to have made a mess of things lately.