(1)6600GT - X1600PRO.
(2)6800 - X1600XT.
(3)6800GS - X800XT.
(4)6800GT - X800XT.
(5)7800GS - X850XTPE (assuming AGP).
(6)7800GT - X1800XL.
(7)7800GTX - X1800XT.
You skipped the 7900GT - X1800XT, but the 7900GT is significantly better.
(8)7900GTX - X1900XT, X1900XTX.
The X1900XTX is only 25MHz/50MHz faster than the X1900XT, easy overclock.