According to a news article from The Inquirer, NVIDIA's GeForce 7900 GTX will be very overclockable.
Officially the GPU runs at 650MHz but some partners will launch the card at 700MHz.
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.
The magical overclocking number is 710MHz and the guys who tested it could not go more than that. The cards are throttling at 720MHz and that's as high as you will reach with air.
This is roughly as much as you can overclock ATI X1900 XTX cards with an air cooler, but as you know those cards can reach a gigahertz if you cool them with dry ice or some other crazy technique. 700MHz or just a little bit higher could be a limitation of the 90 nanometre cards.