8 Pin connector is recommended/required for Proc that are => 125/130 Watts. The i7-920 is listed as a 130 Watt CPU. If you're going to OC (above a mild OC) that baby, I would recommend the 8 Pin. At stock, or mild OC, the 4 pin should be OK.
The diff between the 4 Pin and the 8 pin is pimarily in the IR drop on the individual pins on the motherboard connector.