Beyond3D taking a more detailed look at the two product offerings based on R520, the Radeon X1800 XT and XL, the image quality options they enabled, their performances and where their improvements are.

The X1800 XT is able to open a reasonable performance gap to the other boards here, with it performance up to 38% faster than the previous generation high end board and attain an average FPS a little shy of 60 at 1600x1200. the performance difference between the X1800 XL and X800 XT highlights that the XT's performance is not attained through its excessive core clock and memory bandwidths alone, but through architectural efficiencies as well, as the XL is up to 19% faster than the X800 XT; these efficiency gains mean that the X1800 XL is also faster than the X850 XT PE.

ATI Radeon X1800 Performance and Image Quality Article