Minimum (most games will be unplayable slow)
Windows/Linux OS
CPU: Any that supports SSE2 (Pentium 4 and up, Athlon64 and up)
GPU: Any that supports Pixel Shader model 2.0, except Nvidia FX series (broken SM2.0, too slow anyway)
512MB RAM (note that Windows Vista needs at least 2GB to run reliably)
Recommended
Windows Vista / Windows 7 (32bit or 64bit) with the latest DirectX
CPU: Intel Core 2 Duo @ 3.2 GHz or better OR i3/i5/i7 @ 2,8 GHz or better OR AMD Phenom II @ 3,2 GHz or better
GPU: 8800gt or better (for Direct3D10 support)
RAM: 1GB on Linux/Windows XP, 2GB or more on Vista / Windows 7