Hi there!
Not being a WoW player I had to go see my friends who do play. They all seem to swear by the i5 3570K as WoW only utilizes two cores and no multi-threading, so you wouldn't see any performance increase over the i5 with an i7 unless you plan on doing a lot of other tasks like live streaming while you play.
Long answer short, buy the i5
EDIT: Actually to extend the answer a bit, if you have enough in the budget for an i7, then buy an i5 and use the leftover money to get another 8GB of RAM