You could think about punching a hole in the side that would be inline with the CPU HSF. This would have the fan blow air straight into the HSF which in turn cools the HS and CPU. I have done this and noticed excellent temps under load. Also, this is the only extra fan I've installed. The other fans are part of my PSU. So, it's either this idea or the one you thought of, make a blow hole in the top of your case and have it suck air out (since heat rises).
(Translated from Klingon: Attention Earther!)
that is where my 120 is...and it is so large, that it even blows air on the cpu and KINDA on the ram.
as for the 120 on the top of the case, you should have that blowing out, as heat rises, and you would want to vent the heat out the top, as that is where it should collect.
but hey, if it works...then it works :wink:
Heat does rise up, but it also depends on the ammount of the air your fans circulate in the case. At high volumes heat doesn't have a chance to rise anywhere, it just get blown out of the case. Like my case, the case is ice cold on the top, sides, back.... when running all my fans. Even when runing them at half speed, but I bet the top of the case would be a tad warmer if I would turn off all the fans except the PS fans.
<font color=red>Got a silent setup, now I can hear myself thinking. hmm still silence</font color=red>