I7 2600k w/ GA-Z68-D2H-B3 or i5 2500k w/ GA-Z68X-UD3H

i5 2500k w/ GA-Z68X-UD3H
Because the i5 will overclock well (obviously)
and gaming performance is similar between the 2 CPUs
Getting a better motherboard will result in a higher overclock
and better upgradablity