It really depends on the rest of your system.
If you've overclocked your processer to 3.8, have a brand new top of the line graphics card (GeForce FX or the like), and like to run all the newest games at the max graphics settings with full anti-aliasing... yeah, upgrading your ram to 1 gig will help a ton. But if your like most of us and have a midrange machine, you'll get more preformance spending that money on something like a new graphics card then you will upgrading your ram. More than 512 isnt nessesary unless you want to play the top games at great graphics settings (or you have a job that requires you to have craploads of apps opened at once), but you won't be able to run them that high anyway unless the rest of your rig is top of the line.