Buy a holesaw and a good holesaw blade. Holesaws are just drills, and are pretty cheap and will last a long time, and I cant imagine you dont already have one, and a holesaw blade is...for a good one, like 20-30. DONT get a bad one. If you get a bad one I can pretty much garuntee you'll need to use more than 1 to do 1 hole...They'll dull really quick!
I used 4 of em just to drill 1 80mm in a TV stand I have (needed a hole to run my PS2, vcr, dvd etc wires through).
So get a holesaw and a 80mm bit, and buy:
http://www.newegg.com/app/ViewProduc...125-468&depa=0
or, if thats too big (which it is REALLY big) get:
http://www.newegg.com/app/ViewProduc...125-436&depa=0
I have the second one and its really really great. You can put a 2 fans in the back, 1 in the front at the bottom, and 1 in the 3.5" drive to keep your HD cool. It even has vents on the front so if you put a fan in the 3.5" the air will go in or out unrestricted and quietly.
For the fans, I suggest getting:
15
http://www.newegg.com/app/viewProduc...150-007&depa=0
They move air VERY well for their price! Just $.99 per!!!
That's a great setup imo, and probly the best you'll get for the money (imo).