For GTA IV, you need at least:
CPU:
Intel Core 2 Duo 1.8GHz, AMD Athlon X2 64 2.4GHz
System RAM
Minimum: 1 GB for Windows XP / 1.5 GB for Windows Vista)
Video Card
Minimum: 256MB NVIDIA 7900+ / 256MB ATI X1900+
Video RAM: Required - 256 MB
Video Card 3D Acceleration: Required
Video HW Transform & Lighting
Vertex Shader Ver.: Required - 3.0
Pixel Shader Ver.: Required - 3.0
Free Disk Space
Minimum: 16 GB
Operating System
Minimum: Windows Vista - Service Pack 1 / XP - Service Pack 3