The Radeon HD 6870 and HD 6850 are both excellent cards, and offer an amazing price/performance ratio. In my testing, the Radeon HD 6870 offered approximately 95% of the performance of the Radeon HD 5870, for 64% of the price - all with lower heat, noise and power.
The Radeon HD 6850 performed offered approximately 85% of the performance of the Radeon HD 5870, for 48% of ther price. The HD 6850 vs. HD 5850 comparison held ot that of the 6870/5870 - 95% of the power for 64% of the price.
Six months ago, I said that a Radeon 5870 was the flagship for an Eyefinity setup and should be considered the real baseline for someone looking to really crank through serious frames at max settings. At the time, I also said that I believed the Eyefinity6 card was an even better choice due to the 2GB of framebuffer. Four months ago, I tested the HD 5770 and said that it would make a good entry point into Eyefinity for someone on a budget, especially if you went into it with the proper performance expectations.
I still think 2GB of framebuffer is needed for 5760x1080, and I still like the price point of the HD 5770. The Radeon HD 6850 offers one hell of a performance jump, for close to a Radeon HD 5770 price point. The Radeon HD 6870 offers performance damn-near on par with the Radeon 5870, for the price point of the Radeon HD 5830.
I year ago, the entry price for a high-end Eyefinity setup was $400 (or more, based on demand) for a Radeon HD 5870, and another $125 for an DP > Dual-Link DVI adapter. This was approaching $525 - just for the GPU. If you needed extra monitors, the cost went up significantly. Today, you can get a Radeon HD 6870 for $239, plus $25 for a DP > Single-Link DVI adapter. This puts you at less than $275 for what is essentially the same performance. Saving $250 on your GPU setup is a major improvement in affordability. That money could be put towards a CFX setup, the 3rd monitor, or left in your pocket.
Speaking of monitors - prices have come down significantly over the last year. I am a fan of IPS panels. The colors don't fade as they extend into your periphery, or if you rotate them into Portrait mode. While not a requirement, IPS panels are a nice investment. A year ago, most IPS panels were 24" or larger and designed for graphics professionals - with the price tag to match. The Dell U2410 alone cost $599 each. Today, both HP and Dell have highly rated 1080p IPS panels between 21.5" - 23" for between $249 - $319.
The HD 6870 and HD 6850 are simply some of the best cards I've ever seen or reviewed - powerful, quiet, cool energy efficient and cheap. These are truly amazing pieces of hardware, with a mature Eyefinity ecosystem to back them up. If you have a Radeon HD 5770 or lower, these would be a great replacement with huge benefits. Even a Radeon HD 5830 would see a noticeable increase, but only if you went for the Radeon HD 6870. However, I'd probably hold out for the Radeon HD 6900 series.
The Radeon HD 6900s - now those will be a lot of fun...