the GT is the midrange of 6800 families. Below it is the 128mb 6800, then its the 256mb 6800GT then after that its the 256mb 6800 Ultra. Both the ultra and GT have DDR3 memory and of course are AGP 8x but theres a $100 difference between ultra and GT. I'm going to assume it has to do with the memory bandwidth......After looking it up real quick =P The GT has a memory bandwidth of 32gb while the ultra has 35.2 and the other difference is that the ultra has 2 DVI-I Connectors and S-Video out while the GT has an S-video out, one DVI-I connector, and your normal VGA connector. GT also requires only one spare 4pin power connector to the card while the ultra requires 2 of the 4pin power connectors and I believe nvidia recommends atleast a 480w power supply.
All in all man that GT will kick some amazing ass.