Although I always knew the importance of good air flow in a system, I never could have imagined how important it really is.
I installed my Opteron 170 in my old case (a Rosewill with two 80mm fans in the front and one 80 mm in the back and a side vent/tube pointing at the cpu. I was getting idle temps in the 40-45C range, even at stock speeds. I could not run reliably at 2.5 ghz, and got a couple of high temp warnings during gaming.
I bought an Antec 900, that has the power supply on the bottom, two 120 mm fans in front, a 120 mm fan in back, and a huge 200 mm fan on top. The top fan, with no power supply in the way, pulls heat right out of the cpu. This minute, with the Opteron overclocked from 2.0 to 2.5 (I am slowly working my way up) my idle temps are 24 and 28C per Core Temp. This is with the stock cooler and at stock voltages. All fans are on their medium speed settings. If the included fans are not enough, you can add two more 120's.
This has got to be the coolest case on the planet!
I got mine from Fry's for $99. The description is better at Newegg.