The Intel Core i5-3330S is clocked at 2.7 GHz, but can stretch its legs up to 3.2 GHz if needed.

This is the bottom rung of the ladder as far as processors in the iMac go. You can max out a 21.5" machine with a 3.1 GHz quad-core Intel Core i7 with Turbo Boost up to 3.9 GHz.