Here are the Call of Duty 4: Modern Warfare System Requirements (Minimum)
CPU: Pentium 4 2.4 GHz/ Athlon 64 2800+ (Intel & Athlon 1.8 GHz Dual Core or better supported)
CPU SPEED: Pentium 4 2.4 GHz/ Athlon 64 2800+ (Intel & Athlon 1.8 GHz Dual Core or better supported)
RAM: 512 MB (768 MB for Windows Vista)
VIDEO CARD: 128MB 3d Hardware Accelerated card required-100% DirectX 9.0c compatible and latest drivers (NVIDIA GeForce 6600+ / ATI Radeon 9800 Pro+)
DEDICATED VIDEO RAM: 128 MB
PIXEL SHADER: 2.0
VERTEX SHADER: 2.0
3D: Yes
HARDWARE T&L: Yes
DIRECTX VERSION: 9.0c
OS: Windows XP/Vista
FREE DISK SPACE: 8 GB of uncompressed free hard disk space (plus 600 MB for Windows swap file)
SOUND CARD: Yes
DVD-ROM: 6X DVD-ROM

Call of Duty 4: Modern Warfare Recommended Requirements
CPU: 2.4 GHz dual core or better
CPU SPEED: 2.4 GHz dual core or better
RAM: 1 GB (2 GB for Windows Vista
VIDEO CARD: 3.0 Shader Support recommended. Nvidia Geforce 7800 or better or ATI Radeon X1800 or better
DEDICATED VIDEO RAM: 256 MB
PIXEL SHADER: 3.0
VERTEX SHADER: 3.0
3D: Yes
HARDWARE T&L: Yes
DIRECTX VERSION: 9.0c
OS: Windows XP/Vista
FREE DISK SPACE: 8 GB of uncompressed free hard disk space (plus 600 MB for Windows swap file)
SOUND CARD: Yes
DVD-ROM: 6X DVD-ROM