The higher-end Radeon HD 2900 XTX hasn't been announced yet but FUD Zilla says that by the end of this month we can expect a couple of overclocked Radeon HD 2900 XT cards:
We don't know how much ATI's partners want to push their R600 XT cards but you can easily go up to 850 MHz core with a reference cooler. With water 900MHz+ should be easy but at press time we don't have much details about the cards.
Obviously these cards will end up more expensive and we are not sure about their availability. This will make R600XT, Radeon HD 2900XT look better but most probably even the overclocked card won't be able to beat 8800 GTX at least not in most of the tests.