Hmm, well I don't know if its the best choice for you , but my preferences atm are DPAD, F@H & SETI.
DPAD is for helping to design a new particle accelerator to expand our knowledge of particular sub atomic particles (mostly not covered by the LHC AFAIK). If you want a challenge then I'm no 8 in TA so it'd take you a fair while to catch me their . Theirs only about a dozen of us who are active in this project.
F@H is helping research into various diseases via protein folding. I've got a fairly modest score in this project, so with the right rig (a dual at least) it wouldn't take you long to pass me here , currently I'm only running it on my graphics card.
I think you've heard of SETI , I've got an ok score in this one, mostly it's just my 2nd rig & remote fleet that crunches SETI these days, every now & then I put my quad on it.
Their are stacks of other good projects going on too these days, just a case of whether you have enough PCs .
Btw my 2nd rig is a Sempron(64) 3100 @2.5 GHz, with the optimised SETI client (which uses SSE3 etc) it does WUs in 1-2hrs . Although only a few of the latter variants of S754 Semprons have SSE3 (Venice derived cores).