The 6800 is a "cut-down" version of the GT and the ultra. As opposed to the GT and ultra being boosted versions of the 6800. The 6800 really could be marketed as a different card- the performance is way lower than the GT and ultra- you`ll have noticed from reviews, no doubt. It`s a good card, but I think it would be better saving for a GT. It depends what your using at the moment whether or not it`ll be a worthwhile investment. I think the ATi equiv is either the X800 basic, or maybe the X700, which I THINK is PCI-E only at the moment. Feel free to prove me wrong....
Have been thinkin about a new graphics card too... The FX5950 Ultra gives roughly similar scores to the 6800, but the 6800 looks to be better value for money