Radeon HD 6870 is based on Barts XT while Radeon HD 6850 is based on the PRO version of said chip. You should know this by now, as these two cards are expected to launch on October 22nd, or should we say next Friday.
AMD's faster HD 6000 series cards, the ones that are supposed to replace Radeon HD 5870 and HD 5850, are codenamed Cayman XT and Cayman PRO and if all goes well ATI should launch them in late November.
After that AMD will follow up with dual GPU cards, still scheduled for 2010 launch, most likely December.
We also learned that Cayman is the biggest chip that ATI, or AMD has ever made. It looks like the size of the chip is very close to GF100 aka Fermi and the thermals should be in that range. Judging by our information Cayman XT and PRO should end up pretty fast, much faster than Radeon HD 5870 and 5850 so the guys who already have 5000-series boards should find it a worthwhile upgrade.
This is starting to look like one of the most exciting fourth quarters in years, as naturally Nvidia should launch some cards to counter Cayman.