take it easy, his question was fine. yes, an ati radeon x850 should run bf2 (among other games) nicely will all the graphics options checked.
anyways i have 512 now how much more ram should i buy
what i'm proposing may seem like a crazy idea, but perhaps you can use mathematics to help you answer the problem. believe it or not, the subtraction nessesary can be quite easy. you see, when someone reccomends 1GB of ram and you have half of that, the equation is as follows:
g - y = how much more to buy
in your case, g is the amount of reccomended ram, and y stands for your current amount. it is set up as follows:
1024MB - 512MB = ? (how much more you need)
i'm a busy man so you'll have to do the math yourself, but at a glance i'd expect it to come to about 512 mb of ram. please remember that this is theoretical and may or may not work out in your case. good luck.