Minimum System Requiremetns Left 4 dead:
Supported OS: Windows XP
Processor: Pentium 4 2.4 Ghz or *****alent
Memory: 1 GB
Graphics: 6600 Nvidia graphics card or ATI *****alent with

support for Pixel Shader 3
DirectX Version: DirectX 9.0c
Sound: DirectX Compatible
Hard Drive: 4 GB

Recomanded System Requiremetns Left 4 dead
Supported OS: Windows XP / Vista
Processor: AMD or Intel Dual Core Processor
Memory: 2 GB
Graphics: 7800 Nvidia graphics card or ATI *****alent
DirectX Version: DirectX 9.0c
Sound: DirectX Compatible