Greatly depends on your screen resolution and how high you want to crank up the graphics settings. Another thing to note is that GTA IV is a very CPU heavy game, anything less than a fairly fast quad core will lead to poor performance, even with a powerful video card. The game is also poorly optimized and suffers occasional performance hiccups, even with high end hardware.
If you want high settings at 1080p, something like a GTX 460 1GB or Radeon HD 6850 paired with a fairly fast quad core CPU would probably be the minimum to get good performance.
Honestly, if you have the option to play it on a console, you are probably better off with that option rather than trying to play it on PC.