At default, the 6800 Ultra is marginally faster. The only difference is that the Gainward model is guaranteed (by Gainward) to run at or faster than the 6800 Ultra speeds of 400Mhz core and 1.1GHz memory. A 6800 GT (which is what the Gainward card is) is just a slightly lower clocked 6800 Ultra. Save some money and go with the Gainward. My PNY 6800 GT has far surpassed default 6800 Ultra clockspeeds.
I don't know if nVidia has changed this yet, but the 6800 Ultra requires two 4-pin molex power inputs whereas the 6800 GT requires only one.