Hi Dan,
from my experience you can definitly use all processors that were sold within the available configurations from Apple directly.
These were the configurations sold through Apple:
Intel Core i5 (3330S, 3470S) or Core i7 (3770S) ("Ivy Bridge")
Processor Speed 2.7 or 2.9 GHz (Core i5) or 3.1 GHz (Core i7)
At a maximum you could use the Core i7 with 3.1 GHz.
Hope that helps.