Currently listed System Requirements on the Steam store:
Minimum:
OS: Windows Vista SP2 or Windows 7 SP1
Processor: Intel Dual-Core 2.4 GHz or AMD Dual-Core Athlon 2.5 GHz
Memory: 2 GB RAM
Graphics: NVIDIA GeForce 8800GT or AMD Radeon HD 3830 or Intel HD Graphics 4000 with 512 MB VRAM
DirectX: Version 9.0c
Hard Drive: 10 GB available space
Sound Card: DirectX®-compatible
Recommended:
OS: Windows 7 SP1
Processor: Intel Core i5-2300 or AMD Phenom II X4 940 or better
Memory: 4 GB RAM
Graphics: NVIDIA GeForce GTX 560 or AMD Radeon HD 7750 with 1 GB VRAM or better
DirectX: Version 9.0c
Hard Drive: 14 GB available space
Sound Card: DirectX®-compatible
It's worth noting that even with the following specs, I can *barely* run the game at a reasonable frame rate (looks to be around 25-30 FPS), even with many settings (incl. shadows) turned down:
OS: Windows 8.1 Pro x64
Processor: Intel i7-4770K 3.5 Ghz
Memory: 32 GB
Graphics: NVIDIA GeForce GTX 780 Ti
DirectX: Version 11
"Hard" Drive: 512 GB MLC SSD (only relevant one)
Sound Card: NVIDIA High Definition Audio (GFX card --[DisplayPort]-> screen --> speakers)