I wouldn't worry about the 2900XT being better, the 8950GX2 will put it back in its place. The 8900's are coming after Q3, from what I have heard.
There are benchmarks out. From what iv seen, the ATi is better with DX10 then the 8800GTX, but the 8800GTX is alot better at DX9. But since neither card has good drivers for it, theres really no way to tell until they have been perfected.
Since the HD2900XT's price is nearly identical to the 8800GTS