What will be different is the power use/temps. The 8800GS used very little power and ran cool. The 2900XT was a hot hungry card. You said it was, but make sure your PSU is up to it. No, just because its 600W doesn't mean that it is.
my psu is 500w with 36amp in total over 12v dual rails, this surley could power it. another question is would this card run with 2 6pin adaptors? i read it can use 2 6pins or 1 6pin and an 8pin but i have 2 6pin adaptors.