Depends on resolution and whether aa is used or not.
If you already have an 8800gts 640 you should just overclock it and you will have the same if not better performance than an 8800gt. 8800gt smashes 320mb 8800gts but just beats 640mb 8800gts in many of the tests.
If you want to upgrade rather wait for the 9 series cards.
I upgraded my system in Dec 2007 and purchased a 8800GT, installed it on a dual rail 450W Aopen, and all ran fine. I upgraded my PSU to an 550W ODIN GT over the weekend just to be on the safe side. About the comparison between the 8800GT and GTS...a friend of mine has the 8800GTS running on 3GB ram and XP. Mine, 3GB ram running on DX10 and Vista. I got 160 marks more than him on 3DMARK06 running at stock clocks.
Interresting. Maybe i'm doing something wrong somewhere. Does 10387 sound like a descent score for a E6550, MSI P6N Sli, 3GB DDR800, MSI nx8800GT all running on stock clocks and Vista? Sounds like you know more about this than me...
yeah, seems to be the bottle neck. My board support Quad, so i might have to look into that. The board, as per reviews, is a good overclocker....maybe i'll give that a go first.