The Steam version system requirements are as follows:

Minimum configuration:
Operating system: Windows Vista
Processor: Dual Core 2GHz or equivalent
Memory: 2GB
Hard Disk space: 3GB
Video: ATI or Nvidia card with 512MB RAM
DirectX®: 9

Recommended configuration:
Operating system: Windows 7
Processor: Dual Core 3GHz or equivalent
Memory: 2GB
Video: ATI or Nvidia card with 1GB RAM (Nvidia GeForce GTX 260 or ATI HD4890
DirectX®: 9
Hard Disk space:
     Episode 1: 2.1GB
     Episode 2: 1.54GB
     Episode 3: 1.97GB
     Episode 4: 2.56GB
     Episode 5: 3.05GB
     Director's commentaries: 2.92 GB
     Total: 14.2GB