Not sure on that one, but if you flash it and can't flash back it will void your warranty so its not the sort of thing I'd try. You could have a look at www.nvnews.net for more info. If it was me, I'd save a little more £'s and get a 6800GT. For price to power ratio its about perfect.
Still, the 6800 non ultra has a good bit more punch than my 9800XT which is more ooommph than I need atm.