Left 4 Dead System Requirements
The Left 4 Dead system requirements were revealed in some official box art scans:

Minimum:
- Supported OS: Microsoft Windows XP / Vista / Vista64
- Processor: Pentium 4 3.0GHz
- Memory: 1 GB
- Graphics: 128 MB, Shader model 2.0, ATI 9600, NVidia 6600 or better
- Hard Drive: At least 7.5 GB of free space
- Sound Card: DirectX 9.0c compatible sound card
Recommended:
- Supported OS: Microsoft Windows XP / Vista / Vista64
- Processor: Intel core 2 duo 2.4GHz
- Memory: 1 GB
- Graphics: Shader model 3.0, NVidia 7600, ATI X1600 or better
To easily check whether you meet these requirements, you can visit YouGamers.com which will detect your hardware configuration and let you know if you have to upgrade or not. Fortunately for all of us, Valve designs its games for the hardware the majority of people have, not just for $3,000 systems.
A good measure in terms of real game performance is to check how TF2 runs on your computer. If it chokes up even at low settings, you might be in trouble.

