Ok this is my comparison list it may not be right but this is how i think it goes. Please do correct me if im wrong.
6600 = X700Pro
6600GT = X700XT but it was replaced by the X800 normal nothing else
6800 = X800Pro now with the introduction of the X800XL
6800GT = X800XL
6800 Ultra = X800XT
6800 Ultra OC = X800 XT PE
???? = X850Pro
???? = X850XT
???? = X850XT PE
Neosteve
6600 = X700Pro
6600GT = X700XT but it was replaced by the X800 normal nothing else
6800 = X800Pro now with the introduction of the X800XL
6800GT = X800XL
6800 Ultra = X800XT
6800 Ultra OC = X800 XT PE
???? = X850Pro
???? = X850XT
???? = X850XT PE
Neosteve