I guess i'd have to ask if you mean the 256 or 512mb 7900GT?
If you mean 256, i'd go with the X1900XT 256.
If you mean 512, the it gets a little more difficult. Given the prices of x1900xt 512mbs these days, i think i'd just kick in the extra cash for a x1900xt 512 myself, but since that's not an option, i'd have to see a 512mb 7900GT OC benchmarked with HQ driver settings (which i always use), compared to a an X1900XT 256...