I am stuck in the same predicament - just bought an XFX 8800 GTS 640 from Scan (Lets NOTdebate why this over the 320 one) for a new build for the home gaming computer, but my slightly over eager sibling has ordered a 2900 pro from overclockers... so need to figure out which one needs to go back...
I googled about and the 2900 seems really good value for money - but the noise is meant to be pretty bad (though probably no worse than out current X1800XT). However, if our PSU can take it then it should OC well.
BUT - the 8800 should be fine for all things at the moment, and should be better than AA than the 2900, BUT is more expensive.
And to be honest - I think that BOTH the cards are good for games now, but I wonder if either of these cards will be able to play UT3, GoW and Crysis... Maybe i should just wait for the next set to come out - but we both want to play Bioshock!!
UPDATE: For overclocking - Do you NEED to have both the 6 and the 8 pin power connectors? I've read that the 8 pin one is needed for overclocking - but if you dont have the 8, can you still use a second 6 pin plug? I think that diables overdrive in the CCC, but can you just use Rivatuner or somethink?
Just that when weighing up both, I'm including the OC of the 2900 - which I may not be able to do based on the above - this obviously swings things toward the 8800 again GAH!! I hate technology....