Picture of my liquid-cooled box of 4 Radeon HD5870 cards, before closing up the case and actually getting it to work. It computes about 1.3 Gigahashes per second. (Click to enlarge.)
The side panel that closes it off (not shown) adds another large fan, which I inverted so it's sucking the hot radiator exhaust air out.
UPDATE 5/24/11: I've switched to the Phoenix miner, which somehow gets more hashes out of your card, so I'm now computing about 1.5 Ghash/sec.