If you have the money to afford it, get a 760, although a 770 would be much more future-proof than that. The 660 is a viable choice, but it's probably going to get outdated (and outperformed) soon, so unless you plan on going SLI or unless you're on a strict budget, there's no reason in taking one.
They'll help a lot in Watch_Dogs compared to AMD, and in any other game heavily oriented towards physx (like any ubisoft game except Rayman perhaps...).
If you don't mind about physx, however, taking a 270x (although it is highly advisable to stretch for a 280x) will save you some money and will get you a nice performance in games too.
Games like Watch_Dogs, however, will stutter heavily...I, for one, can't run the game above medium-high with low AA, and I'm on an Asus R9 290x backed up by an i5-4670...
It's up to you in the end, either pay more but play physx games better, or pay less and sacrifice physx where possible.