I'm using the Mn 2 on my i7 920 and have only the stock fan and have found that to be more than enough even when oc'ing to 4+GHz and running 8 instances on Prime95. Never saw it go above 60C. Using a CoolerMaster HAF 932 case. My i7 idels at 36C.
If you are into extreme overclocking, then 4 fans with both sets in a push/pull scenario would be ideal. But even with overclocking, two fans in a push/pull and inline with you case air flow would (should) be more than enough.
The biggest issue I've found with the Mugen 2 is the size. It is a BIG heatsink and, in part, the reason why it works so well.
IMO, the Mugen 2 is (one of) the best bang/buck air coolers available and the stock fan is more than adequate (with proper case air flow).
Hope this helps.