Minimum:

Supported OS: Microsoft® Windows® XP/Vista (Windows 95/98/ME/2000 are unsupported)
DirectX Version: Microsoft DirectX 9.0c (included)
Processor: Intel® Pentium® 4 2.4 Ghz / AMD(R) 64 (TM) 2800+ / Intel® and AMD® 1.8 Ghz Dual Core Processor or better supported
Memory: 512MB RAM (Windows® XP), 768MB RAM (Vista®)
Graphics: NVIDIA Geforce 6600 or better or ATI Radeon® 9800Pro or better
Sound: 100% DirectX 9.0c compatible sound card
Hard Drive: 8GB of free hard drive space
Internet: Broadband connection and service required for multiplayer connectivity

Call of Duty 4: Modern Warfare screenshots