What kind? Anything past 3 GHz and you will need a prett good one.
Right now, I am experimenting with a new system - EP35-DS3P, Q6600, TR Ultra 120 Ext. in an Antec 900 case.
Ambient air temp is 25 C. All the case fans are set on Medium. HSF fan is an S-Flex SFF21F controlled by motherboard.
At 3.3 GHz, Vcore is 1.300 volts, runs 4 instances of Prime 95 overnight. Core temps go to 50 - 55 C.
At 3.6 GHz, Vcore is 1.42 volts, droops to 1.38 volts under load. FSB voltage is + .20 volts, MCH is + .10 volts. Core temps are 60 - 65 C.
Air out the top is 27 C. Air out the back is 28.5 C. Air out the PSU fan is 30 C.
Still experimenting with voltages.
However, I will probably go back to 3.3 GHz.