OP, excellent. ignore the haters.
Haters, Jealousy will eat you alive.
For the temperature of the room have you thought about a cellar cooling system similar to that which you would find in a pub?
The cost will probably far outweigh the benefit, but may give some food for thought.
Couple that with some sort of heat extraction system from the GPU exhausts, as you have eluded to, into the other portion of the cellar.
Might be worth waiting for the non reference coolers on the 290's.
I already have 3 x R9 290 buddy, bought them a few weeks ago. The Mother Mine will consist of mainly 280X cards not 290. I may buy another cheeky 290 just to bring it up to 4
In another forum, someone wanted to put over 30 cards under water, but people said it's next to impossible, as the heat load would be over 15000W.