I hear a lot of people say that S754 is so outdated, but when you don´t have the budged or you don’t want to spend so much money on a new machine the S754 Sempron64 is the way to go. S939 isn’t giving you a performance boost and future upgrades are not that sure with the latest socket (A)M2 witch is also going to have the a Sempron version. Any way, go for a 64 instructions CPU, it most likely will support more optimized instruction sets, for example: AMD Athlon (32bit) does not support SSE2/SSE3, the Sempron (32) S754 does support SSE2 (later revisions also SSE3) and the "F" cored Sempron (64) has SSE3 and x64 instructions too. Also you don’t pay any extra cash, or almost nothing, for a 64 bit CPU compared with a 32 bit.