well a few reasons. 1 clock speed doesnt mean everything. Statistically speaking a 7900GTX would beat the X1900XT, literally it doesnt.
So first. the X1800GTO can be unlocked and OC'ed to X1800XT/XL which is equvilant to a 7900GT
second more pixel pipes. (i think...maybe thats the XT, too lazy to look
thrid the X1800GTO can run HDR and AA, better image quality