I had a 3570k in mine, worked just fine. You can go with a 3770 if you want, but the i7 is a waste for gaming most of hte time. Best overall cpu your going to get is going to be the 3570.
Also, you dont need to get a "k" cpu like I did as you cant overclock it. I got the "k" or "unlocked" cpu since i had plans to move the cpu to a new computer eventually. I did move it and overclocked it to 4.5ghz, I then put a ivy bridge i3 into the x51 along with a 750ti. Runs titanfall maxed out with the i3.