The cards deriving from it will in fact be segmented into mid-range, not high-end as many people expected. So yeah, the best has still to come. The reason? AMD needed to clean up some numbering as next year a lot of new products in the fusion line will eat away the lower segment of the numbering scheme.
The Radeon HD 6800 will segment itself in the mid-range market, whereas the Radeon HD 5800 series will be updated with the Radeon HD 6900 (Caymen) series, with a dual-GPU part named Radeon HD 6900 (Antilles), I can't say it often enough to make this clear I guess.
So that is certainly something to think about, as the naming scheme has changed. Also, why did AMD start with the Radeon HD 6850 and 6870 and not 6900 you might ask. Well, obviously it's easier for AMD to release a slightly lower spec GPU but more importantly, currently the only product that really bothers AMD is NVIDIA's lovely GeForce GTX 460 (GF104). This is a very successful product for which AMD does not have a good answer, as such there's a gap in their product line-up that gap needs to be filled up first.
So with Christmas nearing AMD really needs something out there to, and excuse the language here, cock-block NVIDIA, hence the Radeon HD 6800 series is released first.
Within it's own lineup the Radeon HD 6850 will be faster than the Radeon HD 5830 and the Radeon HD 6870 will be faster than the Radeon HD 5850, but not faster than the 5870. The Radeon HD 6870 will be a pure reference product, while the Radeon HD 6850 will see many custom designs and boards. In the end it will be all about pricing of course.