Ubisoft just released the specs for the PC version of Assassin's Creed III. Note they've only released the minimum specs...anyone looking for recommended, you'll just have to compare your system to these in your head and make an educated guess!
Supported OS: Windows Vista (SP2) / Windows 7 (SP1) / Windows 8
Processor: 2.66 GHz Intel Core2 Duo E6700 or 3.00 GHz AMD Athlon 64 X2 6000+ or better recommended
RAM: 2 GB (4 GB recommended)
Video Card: 512 MB DirectX 9.0c-compliant with Shader Model 4.0 or higher (see supported list)*
Sound Card: DirectX 9.0c-compliant (5.1 surround sound recommended)
DVD-ROM: Dual-layer drive
Hard Drive Space: 17 GB
Peripherals Supported: Windows-compatible keyboard, mouse, optional controller (Xbox 360 Controller for Windows recommended)
Multiplayer: 256 kbps or faster broadband connection
Supported Video Cards at Time of Release:
AMD Radeon HD 3870 / 4000 / 5000 / 6000 / 7000 series or better
NVIDIA GeForce 8600 GT / 9 / 100 / 200 / 300 / 400 / 500 / 600 series or better
Laptop versions of these cards may work, but are not supported. These chipsets are the only ones that will run this game.
No XP support, but then, that's becoming increasingly common. Also, 17GB? For Assassin's Creed? I smell some giant textures.