Ubisoft have confirmed the minimum and recommended PC specs to run Assassin's Creed Unity, and they're fairly demanding.
You're going to need at least a 2GB graphics card - a GTX 680 or Radeon HD 7970 - which is pretty darn bulky for a minimum. If you've got yourself a rig resembling some kind of nuclear reactor then you're going to be fine, but not everyone has a top-notch PC.
The PC specs for AC: Unity are listed below:
64-bit operating system
Windows 7 SP1, Windows 8/8.1 (64bit versions only)
Intel Core i5-2500K @ 3.3 GHz or AMD FX-8350 @ 4.0 GHz or AMD Phenom II x4 940 @ 3.0 GHz
Intel Core i7-3770 @ 3.4 GHz or AMD FX-8350 @ 4.0 GHz or better
NVIDIA GeForce GTX 680 or AMD Radeon HD 7970 (2 GB VRAM)
NVIDIA GeForce GTX 780 or AMD Radeon R9 290X (3 GB VRAM)
DirectX 9.0c compatible sound card with latest drivers
Hard Drive Space
50 GB available space
Windows-compatible keyboard and mouse required, optional controller
256 kbps or faster broadband connection
Supported Video Cards at Time of Release
NVIDIA GeForce GTX 680 or better, GeForce GTX 700 series; AMD Radeon HD7970 or better, Radeon R9 200 series
Note: Laptop versions of these cards may work but are NOT officially supported.
Here's hoping some updated specs that are a little less demanding get posted closer to, or after, the release date.