You wanna max out GTA4 all you need is a decent Quadcore/Core i7 + GTX480/580. It only needs 1.2GB Vram for max gfx with everything 100% @ 1920x1200. Any GPU with 1.5GB Vram should be ok though as its more about CPU Cores than GPU speed.
PC is the only way to play this as intended the console version looks like low setting on PC but even then they are running sub 720P & it still looks terrible with low uneven FPS R* really gave console owners a glitched beta version
Search on here in the screenshots thread for DennisK4 he has some amazing looking mods which make GTA4 look a lot better than the stock settings in some places it looks photorealistic