Processor: Intel Core 2 Duo 1.8Ghz or equivalent AMD Athlon X2 64 2.4Ghz
Memory/RAM: 1GB for XP and 1.5GB for Vista
Video Card: 256MB NVIDIA 7900 or 256MB ATI X1900
Hard Disk Space: 16GB free
Operating System: Windows XP SP3 or Windows Vista SP1
I would recommend you having at least a quad-core (GTA4 wreaks havoc on CPUs), at least 4GB of ram, and a dedicated GPU (I would say a 6630m or better). I wouldn't even bother playing it unless I had at least those specs or better.
More than the hardware, I am concerned about buying it because the reviews on amazon say it has such terrible stability on Windows 7. That it doesn't work well and it's not worth making the purchase. Does anyone agree?