GTA 4 is a beast, i have it on my computer and when i goto the graphic options, it limits itself to the point where it uses ALL my graphic card's memory, things like view distance and object detail. It says something like 870/890mb used
But it does run perfectly smooth with everything else maxed 1680x1050 on my Q9400, GTX260, 4GB ram
Your computer should at least run it on medium settings