Here's a brief pro/cons comparison:
1.) X1950Pro 256 = Fastest card of the bunch, but lame OC'er in comparison to 7900GS.
2.) X1950Pro 512 = As fast as the 256, but the core can't really make use of the 512 memory efficiently, therefore it's useless and price increases for the extra mem.
3.) 7900GS 256 = A little slower than X1950Pro but great OC'er
4.) 7900GS 512 = Same as the X1950Pro 512, good card but the extra memory is really useless.
5.) 8600GT 256 = Crap
6.) 8600GT 512 = Crap with double the memory
7.) HD2600XT 256 = Double crap
8.) HD2600XT 512 = Double crap with double the memory
Yeah, we have a winner, the crappiest card is probably the HD2600XT, but barely, since it's nearly a tie between that and the really crappy 8600GT. It's a choice really of which you want, Green nvidia crap or Red Ati crap.
If you were referring to which is the better performing card that is actually worth anyone's money then that is probably between the X1950Pro 256 (not 512) and the 7900GS 256 (not 512), it all comes down to how overclock-friendly you are.