If you can wait...its always advisable to do so. Sure if you cant play the games you want at an acceptable framerate, then upgrade now...but if you arnt at that critical stage now...waiting a few months may either save you $100 or let you get the next card up for the same $$$.
Typically from a purely economic outlook, its almost never good to buy the top card, but rather buy the card one notch below the top card at a little more than ½ the price. By xmas you may be able to get a Grforce 6800 ultra for what youd pay for a vanilla 6800 right now:aok