Any idea if I would see any performance increase if I moved over from my BFG 8800GTS 320mb to one of these.
I game at 1680x1050 mainly so I think my current card with only 320mb is holding me back on this resolution.
What are your thoughts?
Depends on the game. If it's something like Tom Clancy Ghost Recon then you ought to see a difference with the 512Mb of the 2900 pro as you gain about 30 fps going from the 320Mb to 640Mb GTS however the 320Mb card still does 30 fps so I can live with that.
I find that in 95% of games there is no difference between the 320Mb and 640Mb cards at 1680 x 1050 - well maybe 1-2 fps.

Bioshock 50-70 fps, MOH Airbourne 50-70 fps and even Colin Mcrae Dirt (30+ fps) runs like a dream on my 320Mb GTS and thats everything maxxed out.
Don't forget the memory leak thing is now fixed in the latest beta drivers.
However, I can see you may want to change if you happen to be playing one of the games where you do need more than 320Mb of memory and you aren't satisfied with your performance.
But I would wait for the 2900 pro cards to come out and see how they compare before buying one. It might make the 8800 GTS 640 Mb drop in price anyway to give you an alternative upgrade
However, if it's in most games you feel your not getting good performance then I would suggest it is something else in your system holding you back eg memory, cpu etc.
If you want to post some fps in some games you play, I could compare what I get on my system.