Right, you say your room doesnt have the best airflow yeah? the GTX kicks out LOADS of heat( i mean you couldnt touch it, its that hot) plus you will need a very good PSU for that.
At the moment you have a 19", a 8800GT 512mb will play any new game on or near to max settings on that size screen, then when you get your bigger screen just buy another 8800GT?

I think thats your best bet, as the GT's use less power and they wont kick out as much heat either.

Also 2 GT 512's will be around the same performance if not better than a single GTX, and you wouldnt want two GTX's, your room will be like a sauna