I think the fan that comes with AMD CPUS are usually 70mm. The fan is mounted to the stock heatsink with 4 philips screws so it should be easy to remove the fan and zip tie to the new 120mm heatsink, but the fan sucks so don't do that.
Your case probably came with at least one 120mm fan, so you could use that on the Thor's Hammer. The Thor's Hammer should be able to handle a stock PII without a fan, but chances are you have a fan so use one if you got it. I'd take the fan from the front of the case if there is one there.
If you want to spend the money on another fan, I'd recommend a PWM 120mm fan, since some mobos can't control the speed of standard DC fans.
This is a decent fan, and super cheap. I'd recommend you buy 2 since they're so cheap, and they can be daisy chained.
http://www.amazon.co.uk/Arctic-Cooling-ARCTIC-F12-PWM/dp/B002QVLBM2/ref=pd_sim_computers_2Edited by qwertymac93 - 9/25/12 at 1:32pm