Here are the official system specs needed to play Assassin's Creed on the PC:
Supported OS: Windows XP / Vista (only)
Processor: Dual core processor 2.6GHz Intel Pentium D or AMD Athlon 64 X2 3800+ (Intel Core 2 Duo 2.2GHz or AMD Athlon 64 X2 4400+ or better recommended)
RAM: 2GB (3GB recommended)
Video Card: 256MB DirectX 10.0-compliant video card or DirectX 9.0-compliant card with Shader Model 3.0 or higher (512MB video card recommended) (see supported list)*
Sound Card: DirectX 9.0 or 10.0 compliant sound card (5.1 sound card recommended)
DirectX Version: DirectX 10.0 libraries (included on disc)
DVD-ROM: DVD-ROM dual-layer drive
Hard Drive Space: 12GB
Peripherals Supported: Keyboard, mouse, optional controller (Xbox 360 Controller for Windows recommended)

*Supported Video Cards at Time of Release:
ATI RADEON X1300-1950 / HD 2000 / 3000 series
NVIDIA GeForce 6600-6800 / 7 / 8 / 9 series

Laptop versions of these cards may work but are NOT supported.