Nvidia Geforce 7900 cards are very overclockable
 

WE learned that many manufacturers will overclock the stock 650MHz Geforce 7900 GTX cards. The cards are at the various labs and Nvidia partners are working on their final clocks.

Everyone wants to clock those cards to 700MHz and some of the partners might actually be able to do it, ship it and to support the warranty at such a high clock speed.