I'll second what twite is saying, but it is a matter of personal preference of course.
You'd be going from a VERY good card to a great card. Is that 6800GS already starting to slow things down for you? What games are you playing, and at what resolutions?
If you are a hardcore, fps maniac, then sure go for the upgrade. If you are just a casual gamer like myself, that 6800GS still has plenty of life in it.