GTA 4 is a very poor port of a console game. A quad core CPU is recommended, dual core CPU is considered minimum, and a single core CPU means you are out of luck. That means the game is very CPU limited.

Back when the game came out, I remember reading complaint from someone who had a powerful quad core CPU (Q9650??) and 8800GTX video card. Standing still he only managed to get 20 FPS @ 1920 x 1200 resolution.

 
This chart should should you just how CPU limited GTA 4 is, its at 1280x1024 and only medium settings and not a single CPU was able to raise the FPS over 63 on average.
http://www.tomshardware.com/charts/2009-desktop-cpu-charts-update-1/GTA-IV-1.0.3,1402.html

Here is a chart from the 5870 release article showing you the performance levels of various GPUs are various resolutions, note that the most powerful setups hit a CPU bottleneck
Grand%20Theft%20Auto%20No%20AA.png


If you plan to play GTA 4 at highest resolution and settings you are going to need a top of the line rig, not because its a good test of hardware like crysis, mostly because its coding sucks.