Disclaimer: right now, everything is "rumor" - until proven a fact, please keep this in mind.
For what has seemed like a very long time - AMD has teased us with no refresh of their Evergreen (5xxx series), then the name change from ATI to AMD and now finally, some concrete release dates for you to check on your calender.
October 18, 2010.
This is the day AMD will unveil the new, now AMD branded, Radeon 6850 and 6870 cards. The 5k series was a great card for performance per watt, which has been AMD's strategy with the last generation of GPU. This generation kicks it up a few notches...
The 6870 will compete with the current 5850.
To compare: a 5850 has 1440 stream processors, 725MHz core, and uses 151W of power. The 6870 will beat the current 5850, while only using 960 stream processors, an increased 850MHz core and uses >150W power. While it uses more power, it has cut down 30% of it's stream processors and can still beat a 5850... it will be interesting to see what a full 1400+ stream processing GPU from AMD can do.
AMD are using the 6870 to replace the 5770 - this is where it gets confusing. AMD are changing naming schemes... the 6870 will replace the "5770-esque" part of the market, while the new 69xx series will replace the current 58xx market-segment.
From what I've seen - it looks like AMD have shoved an NVIDIA GTX460 into a dog's face, and made them chase it - the hunt is on, AMD want that GTX460 segment and if they can pull off 5870 performance for just over 150W and $200 - they'll have it, in droves.