Jonathan Morin, the Creative Director on Watch Dogs recently Tweeted out the minimum PC requirements for the game. It does seem to indicate that the operating system needed for this game will have to be 64-bit.
Intel Core 2 Quad Q8400 2.66Ghz or AMD Phenom II X4 940 3.0Ghz
6GB RAM
1024 VRAM DirectX 11 Shader 5.0 capable video card
DirectX9 capable sound card
25 GB free HDD space