I used water cooling. With 5970 it do not adds too much to price, but helps keep cards cool in tight environment. Specifically, I had microATX case with two 5970 at 850 Mhz (it could handle more, but just wanted to feel safer). GPUs were staying at 55-65 C. I bet you can do better if you route separate water tubes to each card (which I did not because of tight environment and being lazy :))

Before adding water cooling - it would lock after 30 minutes of mining at standard 725 Mhz.

This system worked very well for ~4 months (and then I sold it to guy from Alaska, since it became too hot in Texas :))