The Space Marine Forums offer the official Warhammer 40,000: Space Marine Windows system requirements, which include a Steam account. The minimum and recommended stats follow.

Minimum Configuration

• OS - Windows XP SP3, Windows Vista SP1, Windows 7
• Processor - 2.0Ghz Dual core CPU (any Core 2 Duo or AMD X2 or better)
• RAM - 1GB (XP), 2GB (Vista), 2GB (Windows 7)
• Hard Drive - 20 GB space free (10 GB free after install)
• Video Card - 256MB Video Card using Shader Model 3 & DirectX 9.0 or better (Performance equivalent to an AMD Radeon 3850 or NVIDIA GeForce 8800GT)
• Online Steam account

Recommended Configuration

• OS - Windows 7
• Processor - Any Quad-core AMD or Intel Processor
• RAM - 1GB (XP), 2GB (Vista), 2GB (Windows 7)
• Hard Drive - 20 GB space free (10 GB free after install)
• Video Card - 512MB Video Card using Shader Model 3 & DirectX 9.0 or better (Performance equivalent to an AMD Radeon 5750 or NVIDIA GeForce GTX 260)
• Online Steam account

Disclaimer: Minimum specs are subject to change