have just installed GTA 4 in my system, and its running like a cowww, i've tried it on lowest graphics setting..... still very slow, Here is my system config...
i5 intel processor with 2.90 Ghz
8 GB DDR3 RAM, 1TB Hard Disk
Asus NVIDIA GeForce EN210 1 GB DDR3 Graphics Card
If money is no object then a GTX 680 or if you board supports SLi then 2 x 680s but its more than you really need a Radeon 7850 2Gb will be more than enough for that title. Note the game is poorly optimised for PC and can run badly on the best hardware.