The X1K series is hotter than ever. Its using the most voltage of any video card out there and despite its controversial cooler, it stills runs as hot as ever.
As for the 6800GT vs. X850XT, its pretty much give or take. In some games, the 6800GT leads by a large margin, and in other games the X850XT leads by a large margin. Usually, its the 6800GT that leads and I would choose it over the X850XT just because of OpenGL and SM3, but the X850XTs handle 1600x1200 resolutions better. Though the rest of your hardware had better be damn good too to be running at 1600x1200.
As long as you can keep the card, processor, and basically the rest of the case cool, then its fine to have whatever you want inside it. Be warned though; My friend bought a semi-shuttle. It was a shuttle but wasn't called a shuttle. Anywayz, two wires in the Power Supply connected, causing a huge surge. It destroyed the PSU and literally fried half the motherboard.
EDIT: Oh yea, and the 6800GT is MUCH cheaper than the X850XT. If you can get the X850XT on "cheaper-than-shelf" price then you should also be able to get the 6800GT on "cheaper-than-shelf" price.