Well, DX11 doesn't have the most demanding features dude... it's really not that big of a step up, that is if you exclude tessellation (which is scalable, so you could probably set it to x2 or x4 with the 5770)
Honestly, I don't know whats going to happen in the next to months. So in January AMD might release an HD5830, or they may never... they might price drop, or they might hold out for a very long time.
Nvidia's Fermi is looking like ghostware at this point. It might be the greatest thing since sliced bread or it might be like ATI's 2000 series. All I'm saying is that get an HD4890... it's plenty powerful (stronger then an HD5770), and remember that DX11 isn't a major improvement over DX10. it will take a year to a year and a half for game developers to accurately implement DX11, at which point you may actually want to buy a new gfx card. Remember that it's also unlikely that the DX11 featured on the HD5770 will make any difference in terms of graphics because it will run out of steam very quickly (long before the HD4890 will).
Also, I realized that the HD5770 would be far more effective if AMD gave it a 256bit bus... would be a little faster then the HD4870... but alas...