Anyone who could advice what fan to use on the megahalems ? I was thinking about getting a scythe PWM.

Im using P183, cooling an I5 2500K.

Also for the case I was thinking about getting 2 slipstreams 800RPM and undervolt to 5volt. Would it be better to get another one for the top because of the sleeve bearing?

My Mugen 2 came with the 'medium' version of their PWM case fans. Based on its specs it's the best they make for a silent PC because its maximum decibel rating is in the high 20's and it keeps my Phenom II x6 plenty cool without getting annoying.

I'd also get a vibration dampener, they help a lot.

Scythe Slipstreams are good. But you don't really have to undervolt them that much, I have a 1200 rpm version running on 600 rpm and it is silent within my case (FT02).

I'm about to assemble the same system as you in the P183 aswell.
Can I ask you why you choose the Slipstreams over SPCR's reference fan Nexus 120? It seems to me like it's quieter.

No, actually they shouldn't be quieter: the Slipstreams are nearly identical to the Nexus ones, maybe even a bit quieter (running 50-100rpm less at 5V, while giving the same about 13 CFM, even if I dunno about relevant static pressures).

Yes I concur, they are both good fans.

It would seem that, generally, from the testing results attained by SPCR over time, that it is likely the Nexus has better static pressure than the slipstream... but this is wild guesswork/extrapolation on my part.

The slipstreams clear advantage over the Nexus is that they downvolt easily to 5 volts. The Nexus can't do that.

At 13 CFM, static pressure is not much of a factor... the airflow is more like drifting air than a stream of air. It is like that aerodynamic efficiency of a car - very important at 120 miles per hour - but much less so at 12 miles per hour.