Little information about the 55nm GT200b parts has leaked to the press but according to a post at the VR Zone forums a 55nm version of the GeForce GTX 280 will be released at NVISION next week. It's unclear whether this will be an actual product launch or a paperlaunch.
The biggest technological difference between Radeon HD 4870 X2 and Radeon HD 4850 X2 is the memory type. The former uses bleeding edge GDDR5 memory circuits, while the latter uses GDDR3. This means that it will have about half the bandwidth, which will limit performance somewhat. It also has a lower core frequency, 625MHz vs. 750MHz, which makes up the biggest performance difference between the two. However, there is always the option of overclocking.
Performance is expected to be slightly lower than 4870 X2, but still very competitive with NVIDIA's enthusiast cards of the GeForce GTX series. Right now it looks like Radeon HD 4850 X2 will be priced to compete with GeForce GTX 260, while performance should be a notch or two higher. One more month to go before the card surfaces.