7800GTX-512MB > X1800XT > 7800GTX-256MB > 7800GT-256MB > X1800XL.
The 7800GTX-512MB is by far the best, MUCH better than the X1800XT.
The X1800XT, however, is A BIT better than the 7800GTX, but is $100+ more.
The 7800GT is better than the X1800XL, but the X1800XL is still mroe expensive.