First is first. Just get the GT. Even at stock speeds, the GT is not that much slower than the Ultra. The difference IMHO is not worth the extra $100 or so you'll spend on an Ultra.
As far as differences are concerned, the 2 cards are virtually the exact same. Same GPU, same PCB, same I/O options, etc. The real difference comes in clock speed, HSF (heatsink-fan) size, and power requirements. You see, in AGP, the Ultra's require 2 molex connectors, while the GT's only need 1. On PCI-E, however, both require just the one PCI-E graphics power connector (the little 6-pin power connector). The Ultra draws a bit more juice than the GT, but for the most part, its negligable. IMHO, the biggest difference between the 2 is the HSF size. The Ultra's come with a stock mongo 2 slot cooling system, while the GT has a 1-slotter. I took the liberty of intstalling an Arctic Cooling NV Silencer 5, which turns the card into a 2-slotter, but that was not of importance to me. The only thing that I would think the 1-slot cooling system would affect on the GT is the overclocking performance, but again, that can be changed with an NV5. Finally, there's the difference in clock speeds. GT's run a core speed of anywhere from 350MHz - 370MHz (depending on manufacturer), while Ultra's have core clocks of anywhere from 400MHz - 425MHz (again, depending on manufacturer). The Ultra Extreme versions are EXTREMELY rare, but I believe they run a 450MHz core clock. The memory on the GT is clocked at 500MHz (1.0GHz effective) for most (if not all) cards, and the Ultra's memory runs at 550MHz (1.1GHz effective).
In terms of being able to overclock a GT to an Ultra, that depends on the card. Oftentimes, when a company makes its "high-end" chip, some of the chips can't run at the uber high speed that others can, and these "defective" chips are oftentimes given slower speed ratings (such is the case with CPU's sometimes). These chips are physically OK, but can't run at maximum speed. The NV40 GPU, which powers both the GT and the Ultra, is exactly the same GPU on both cards, with no difference whatsoever. I believe that some of the GT's are simply Ultra's that couldn't clock to the speed of an Ultra, thats all. You'll just have to try to overclock the card to see what speeds the card can run safely at, and go from there. Maybe you'll get Ultra (and sometimes more) speeds, sometimes you won't. It really is "the luck of the draw". Not to dishearten you, but in my net travels, I have seen about 95% of 6800GT users be able to overclock to at least Ultra speeds, and again, sometimes beyond.
