They are both based on the same core so assuming neither is overclocked there will be no difference between the cards except maybe the cooler. The only difference will be the companies warranties and RMA policies.
I know that Sapphire had a lot of heat problems with the X1950 that other companies didn't seem to have. It shouldn't be an issue though.
Go to each manu's website and check out their warranties. If they are similarly priced, go with the best warranty.
yes, the warranties isn't that matters right now
I've navigated for a while and find the 2900xt with serious heat probs compared with the 8800gts.
I think i'm gonna get me the 8800 one instead of 2900xt, anyway , thx everyone.