Although this is true for most games but some games like Crysis can use 800 mb of ram.512mb is more than enough for today graphic cards. You will not get any gain in performance for any extra mb over this, and if you get some, it will not worth the extra cost. Better to look for the highest ram frequency, it will give you more gain than extra ram.
edit: Like SugarJ said, it's true that it depends of your screen size, but under 24", I think more than 512mb is useless.