Right now at this time, You would need an i7 4960x and a GTX 780 ti to run Watch dogs smooth at ultra settings @1080p with no AA.....But After updates and better drivers I really would hope that it runs better. But like JOOK-D said a mainstream Socket like LGA 1150 is great with an nice quad core. The recommended Intel CPU is the i7 4770k on a few sites i looked at. And the recommended AMD CPU is the FX 8350....and one site says the FX 9370
Looking at the review, this game will use all the cores you have at your disposal on Ultra. My next rig (I think) will be the 6 core Haswell-e - are you in a panic to build or can you wait/afford the Haswell refresh?
-Bruce