If youre running XP, 512Mb is pretty good. It all depends on what applications you are using and the other components in the computer, because a really good computer with 512Mb could use an upgrade. If you are playing alot of games and memory intensive appliactions, 1Gb would help a little. RAM is getting cheap, another 512 would only cost $40ish. Also, check your RAM speed, that will also effect the performance.

-C