well please dont start the discussion once again..the 2900xt vs 8800gts/gx battle
ok ill put it in short
IF YOU think the future Catalyst releases will put the 2900XT ahead then you can take the risk. Noone here or anywhere can guarantee that.
At the moment i would take the 8800GTS over the 2900XT, bvecause the current benchies put it ahead or at least make it a much better VFM since its cheaper than the 2900XT, and has much more stable drivers.Other than that, it requires a comparitively lower PSU, and gives out much less heat, but that wont be much of a concern.
Its all up to you. You wanna gamble, you can go ahead and maybe youll hit the jackpot in the coming months.