Counter-Strike: Source
Your system must meet these minimum requirements to be playable:

Minimum:

OS: Windows 98SE/ME/2000/XP
CPU: 1.2 GHz Pentium 4 or AMD 1200+ Athlon processor
RAM: 256 MB
Hard Disk Space: 7 GB available for full Steam
Video: 32 MB DirectX 7
Sound: Stereo Speakers/Headphones
Multiplayer: 56kbps internet connection

Recommended:

OS: Windows 2000/XP
CPU: 2.4 GHz+ Pentium 4 or AMD 2500+ Athlon processor
RAM: 512 MB+
Hard Disk Space: 7 GB available for full Steam
Video:128 MB DirectX 9
Sound: Stereo Speakers/Headphones
Multiplayer: Broadband Internet Connection