If you do a lot of DivX or some graphic intense programs, then 1Gb will help. You can also turn off in most cases or lower the swap file to 64Mb or less with 512Mb and for sure with 1Gb. This will force windows to use the faster ram and not the slower swap file.
if you don't go into swap file much. then no, you wouldn't see much of a difference. BUT, i have seen battlefield 1942 use 360+ megs of ram. so its foreseeable that many future games will use over 512 megs of ram.