graffix

Hi all, My current rig; (nothing overlocked) 2GB ram Intl 2.4Ghz Dual Core nVidia 8800GTX Win Xp 32bit Im just wondering how much of a difference I would see/gain if I were to upgrade the RAM (4GB) as well as the gfx card, to something like an ati 5870. At the moment, it's running smooth on 'high' with a couple on 'normal' at 1680X1050 with AA and shadows off but stuggles a bit on missions with tons of AI. Or would getting more RAM + Win7 be recommended? thanks!