As lots of others have said, I would really check out where your current memory usage is coming from before you decide to go out and buy new memory.
As a few RPG-type game references (not sure about other genres
), when I played WoW --- which is not that graphics intensive but is fairly memory intensive --- my total memory usage rarely went above 700MB. So if you had only 512MB of RAM an upgrade to 1 Gig helped a lot, but beyond that the RAM was simply wasted. Oblivion also did absolutely fine with 1 Gig (but was a lot more demanding that WoW on the Video card), as did Titan Quest.
The first game I've ever played that does use more than 1 Gig of RAM is Gothic 3, which needs 2 Gigs to run smoothly because it preloads the graphics for environments to avoid loading times or zone boundaries (and probably because it's among the most inefficiently coded games ever made).
No doubt in the future more and more games will need 2 Gigs of RAM to run optimally, so maybe buying extra RAM wouldn't be a wasted investment for gaming if you plan on keeping your current machine for a while, but maybe it's totally unneccesary and it's not a lack of RAM that's causing your problems.