I am about to order 10 of these 80MM fans for $12.90 shipped    .
http://www.newegg.com/Product/Prod [...] 6835150007

My current XION Onyx case has 2 120MM fans and 1 140MM fan, however, they are unbelievably slow RPM.
http://www.newegg.com/Product/Prod [...] 6811208021

I might get a HD 4850 to upgrade my 7900 GS, but under load, because of their single slot cooler, they run really hot (85ºC is "normal"!). In my opinion, that's unacceptable.

Ok, that leaves 5 fans left. I will probably jam a couple into the hard drive bays just to keep airflow going over there. Would I be able to drill some grills into the top of the case and put a few there? It would seem that air just pools there since there aren't many fans there. Come on I got 10! I want to use them!

Can someone help with this? What about drilling through the other side (not the window)? lol

I would try to get a second one besides the 80 and under the 140 and make sure the 2 front are pumping air faster than the side ones are that way the air that comes from the side is pushed from the ones in front and not the other way around, also put a few on the bottom (make a sort or windtunnel stand for your case) an then put the rest on the top.

If I were to try and drill into that acrylic, what would I have to do? Obviously it would be different then drilling into wood.

the best thing you can do is jam 4 fans on the top and stuck the 1 out of the 5 at the buttom.and take the one under the 140mm to the bottom also.this way you will have a massive airflow toward the top which is how the heat will travel.
 
because side fan really ruins the overall airflow inside the case.so might as well convert the airflow upward completely but still leave some airflow out the back.

 
Actually what he said sounds much better, the more different directions there are the more jumble the airflow is.
 
Come to think of it, wouldn't it be smarter to have all your airflow going down to up? The only problem I can think of would be ther videocards being slightly in the way...

In my experience, side fans brought my temps up by 3C. I have a CM stacker 832 with the 4 side fans. Removing it one by one decreased all my temps some. I only can imagine that the way the fans were designed didn't like other fans blowing on top of them.

but his case dont have so much fan on the side.
 
its 2 way of looking at it. you can cool the whole system in a brutal way of just blowing tons of air over it which is what evilonigiri is saying. or 2, make a actual airflow inside the case which in his case he got enough space at the top so he fit 4 at the top and maybe 2 or even more at the buttom so he can cool the hot VGA card as well.

If you have a spare Intel CPU cooler you can take the fan off, and the pushpins wrap around the back I/O slots under the vid card. I have the green version of this case, and am using that for extra outtake for my 8800GT. IMO all those 80mm's will make it whine really loud... the smaller the fan the less intake/output, and the more noise to try and achieve more intake/output.

brendano he already bought all the fans. i dont think you comment on the noise and intake/output will help him much!