The reference ATI card had a fan in the back that pushed air out to the back of the case. The current designs by XFX, PowerColor, VisionTek, Sapphire, and I think I'm forgetting a manufacturer will usually place the fan in the middle for some odd reason (I guess to increase cool airflow to the core?). I'm sure it works in 1-2 card configurations, but as you have said (and I've seen this myself), when stacked the airflow is crap.
I believe the 6970 still has the fan in the back on most manufacturers, though be sure to get detailed pictures of the actual item to confirm. I'd recommend Sapphire if available, and stay as far away from XFX as possible.
There is always water cooling if you can't find a suitable card, though this will add ~$300-400.