A German gamemagazine called "Gamestar" has testetd GTA 4 on PC:

The result:

You will only be able to play on low graphics with a geforce 7900 gtx or gt2!! confused.gif
On older cards the game crashes!! and on pentium d's and pentium 4's too!
the magazine dont tested it with ATI cards because the game crashed but rockstar said that they will fix this problem!
On older card then a geforce 7900 it maybe will run on 800x600 or lower. on 1024x786 on a geforce 7600 it laggs like hell they say!
The graphics on low are looking like the graphics on the consoles.
The minimum required processor to play the game fine is a amd athlon 64 x2 4000+ or a core 2 duo with the same speed. If someone hs a geforce 8500, the game wil crash!
Remember that Gamestar only tested all low end cards with 1024x768 and not with 800x600 or 640x480!!
they had an older version of the game were ati cards were not supported but in the final version they will be supported<!