Minimum:

Microsoft Windows® 2000/XP. (Windows® 95/98/ME/NT not supported)
Pentium IV 1.4GHz (or AMD AthlonXP 1700+ processor or higher).
256MB System Memory.
3D Hardware Accelerator Card Required - 100% DirectX® 9.0c compatible with 128 MB and Shader model 2.0. (GF FX 5700 or ATI 9500)
100% DirectX® 9.0c compatible 16-bit sound card and latest drivers
4.0GB of uncompressed free disk space (plus 600MB for Windows® 2000/XP swap file)
100% Windows® 2000/XP compatible mouse, keyboard and latest drivers.

Recommended:

Pentium® IV 2.8GHz with Dual Core Technology (or Athlon 64 series).
512MB System Memory.
3D Hardware Accelerator Card Required - 100% DirectX® 9.0c compatible with 256 MB and Shader model 2.0. All nVidia® GeForce™ 7 series or higher.
Sound Blaster® X-Fi series or higher.

Optimal:

Just Cause