Buying more memory isn't a solution. Something is terribly wrong with your comp. I have built 6 comps in the last 6 months, 4 of them with 3200+ A64, and 1 gig of ram, with varying video cards (6600, 6200 turbocache, 7600GT). All of them can play BF2 without any lag, as long as the graphic settings are right for the card. ie the cpu isn't making any lag, and the 1 gig of ram doesn't matter THAT much. If you turn your settings up to medium-high, then yes, you may need more then 1 gig of ram. However, in your case, you have a problem that isnt related to the amount of ram you have, because you say you get lag even on 800x600 and no AA of AF.
Don't waste your money on RAM until you figure out what the problem is.