Also, you would definetly need a beefier CPU.
GTA IV is more of a CPU intensive game than graphics, although a good graphics card helps, especially as Yowan just stated but it does need a beefy CPU as well, especially if you are trying to play the game on highest settings.
For the highest possible settings, you will need either a top Nvidia card (GTX 480 or 580) or an ATI card with 2GB.
Lucky