Wait for a year or two when games use dx10 and tessellation engine(which the 8800 doesn't have). The 8800 will pretty much be chugging, while the 2900xt is still kicking strong.
Where do you base that...the GTX is WAY more powerful than the 2900XT...and with time the XT will show it's age. If the GTX is already more powerful, how can you say that the XT will beat the GTX in the future.