If your not buying untill christams then the new ATI cards (Southern Islands) will be out anyway so you wont have to wait and will know if they are worth getting or not.
Buy now or wait is never cut and dry.
To me it depends on your situation and what you know is coming.
We knew when the 5770 was due out and what to expect roughly in both power and performance terms.
Now if someone had posted about getting a card for a 22" display back then you could have pointed them at a card we knew would be slower in the 4850 or at something similar like the 4870.
But that card needed 2 6 pin PCIE cables and not everyone has two, so it could have meant a PSU upgrade.
So that would make it plain stupidity to just buy what was out now and not wait for the 5770.