You have a 5870 now? You change cards a lot
I'm in a similar position, although I think I'll definitely be going for the 7900 series. 7800 could be great for the money, 6970 performance (maybe more) at around $200 and lower power would be great. What's mainly up in the air there is how much lower power 28nm allows for. It should be somewhat like a 5770 compared to a 4870 I would imagine, although maybe a little more as they aren't cutting the memory bus in half this time.