Unless overclocking I wouldn't bother. If you are keen however (note I've not looked at an MDD for a while - assuming it is that), see how much clearance is between the heatsink and power supply. Go for the biggest, slowest fan you can find that will fit - any 12V one will do, and it could be attached by various means - automotive double sided tape, self-tapping screws, etc.
The last version of GeekBench (v2) is still PPC