Thanks for your answers. I'll stay with gtx 460.
I have 4gb of ram, 3.3gb of which is in use, because windows are 32bit.
The thing is that gta 4 ran fine with my old graphics card, of course on low settings and with low frame rate, but there wasn't that constant dropping to 0 fps. During benchmark I get 30 to 50 fps, but while playing game stops every two seconds. Not that I am very eager to play it, because I already did, but that strange issue got me kind of worried that other games might have same problem.