Have a 6600GT 128MB AGP running on my motherboard atm as an "upgrade" to my old Radeon 9500pro. There are definitely some new effects and improvements, but the framerate difference isn't dramatic (I believe my aging CPU is the bottleneck now, an Athlon 2500XP Barton core).
From a price standpoint, 6600GT is a great value and will probably "last" two more years (depending on your desire to play the latest and shiniest games).
So if you have a decent CPU that can push a 6600GT, that'd be a good purchase. If you have an older CPU, you'll only notice a few shinier effects (depending on what you're upgrading from) and minor framerate improvements.
If you have a great CPU, 6800GT is the way to go.
If your motherboard and CPU are falling behind the curve, then probably consider grabbing a new set of toys (after saving up some renown with your wife.). So up the tonnage, sir!
