if you want something that will test the system as a whole i find 3dmark2001 to be very good if you loop it...it seems to be very sensitive to video card, memory, and cpu overclocks. if you want to isolate the cpu, i think prime95 is the most stressfull, it seems to find errors that don't show up with any of the other popular distributed computing programs (i.e. seti, ud, etc...). the thing with distributed computing tests, however, is that you can't really run those AND something else (like quake3, 3dmark, another distributed computing program, etc...) because they are meant to use the unused cpu cycles. basically, lets say you run seti and rc5, one of them will take 100% load and the other will "yield" because there doesn't seem to be any unused cycles (the other program is using them all).