Acordding to the inquirer nVidia is planning on releasing a 512MB model of their 7800GTX in order to
compete with ATi's next-generation 512mb cards (such as the X1800XT and X1800LE).
Eventhough The Inquirer isnt known for their most truthfull stories I still believe a 512MB model of the 7800GTX
is bound to show up someday.
Source:
http://www.theinquirer.net/?article=26595