Yea, ATI held back the X1900XTX clock speed so if you used it with a Crossfire master card it wouldn't be unstable, or something. I also don't think there is a X1800XTX, there is just X1800 XL and X1800 XT.
I would put ATI's counterparts in as well as which is better in terms of performance (note: I discluded biased games like Doom 3 and HL2)
7300GT < X1300pro
6600GT > X1600pro
6800 < X1600XT (performs better with AA, but same without)
6800GS > X800XT
7600GS > X1600pro/XT
7600GT > X1600XT
7800GS > X850XT PE (AGP)
7800GT > X1800XL
7800GTX = X1800XT (Around same power, GTX still a little better)
7900GT > X1800XT
7900GTX = X1900XT/X
Those are from articles that I've read before, so things might have changed or I forgot something. Also remember the X--- series only support SM 2.0 where as the Geforce 6/7 and Radeon X1000 series supports SM 3.0
Some cards are meant to replace older cards with newer technology and not necessarilly win over any benchmarks (some may be better)
Replacement - Older card
7300GT - 6200
7600GS - 6600GT/6800
7600GT - 6800GS
7900GT - 7800GTX
X1600pro - X800/pro
X1600XT - X800XT
I hope this clears up some confusion. I didn't go through all the reviews so some may be different, but I think they're pretty accurate.