That depends on what you're running. I went from 512mb Crucical CL2 PC133 to 512mb Crucial PC2100 with my (then) 850mhz Duron+K7S5A. Got about 800pt boost in 3dmark2001, roughly 10 fps in Q3A (and similiar Q3A based games), but ONLY 1-3 fps in UT (D3D - ditto with games using Unreal engine). Was it worth it? Probably not if I had to pay for it, but I already had the PC2100 left over from another system I upgraded so it didn't really cost me anything to install it.