The minimum requirements:

Windows 7/Vista/XP PC (32 or 64 bit)
Processor: Dual Core 2.0GHz or equivalent processor
2GB System RAM
6GB free HDD Space
Direct X 9.0c compliant video card with 512 MB of RAM
DirectX compatible sound card
Internet access for Steam activation

And the recommended requirements:

Windows 7/Vista/XP PC (32 or 64 bit)
Processor: Quad-core Intel or AMD CPU
4GB System RAM
6GB free HDD space
DirectX 9.0c compatible NVIDIA or AMD ATI video card with 1GB of RAM (Nvidia GeForce GTX 260 or higher; ATI Radeon 4890 or higher).
DirectX compatible sound card
Internet access for Steam activation

And yes, we're thinking 6GB of disk space must be wrong too. Maybe they mean 16GB?