1) Asus GTX770:
- has a backplate, great customer feedback.
- I did a lot of research on all the GTX770's and think this one is the best.
(Since you'd likely get Watch Dogs please note that if you crank settings to Ultra not only will your frame rate be lower than I recommend but you can get STUTTER by maxing out the 2GB of Video RAM. You're fine if you tweak properly. NVidia and others have guides.)
2) GPU clock speed:
Yes, it makes a big difference. An 1100MHz GPU can process up to 10% faster so in theory COULD produce 10% more frames per second.
http://www.techpowerup.com/reviews/ASUS/GeForce_GTX_770_Direct_Cu_II_OC/