According to a post on Fudzilla, the RV670 won't be out until early 2008, despite all sorts of rumours that it will be out in time for the Christmas holiday season. Not only that, but the RV670 was designed for 65nm, although it's now looking like it will be moved to 55nm before it enters production. The RV670 is the missing piece of the puzzle right now, as it fits the space between the GDDR4 version of the Radeon HD 2600 XT and the Radeon HD 2900 XT. The RV670 should have a 256-bit memory controller and will as such replace the Radeon X1950 Pro.
The advantage of moving to 55nm is a smaller and cooler running chip, as it will draw less power, which means less heat. Again, according to Fudzilla, we'll see dual RV670 cards appearing, although we're still waiting for dual Radeon HD 2600 XT cards to appear in retail. A dual RV670 card is meant to outperform a GeForce 8800 Ultra according to Fudzilla, although the question is how cost effective this solution will be.
Fudzilla also claims that there won't be any other cards this side of Christmas from AMD, although some websites believe that a shrunken version of the R680 will be available in time for the holidays. Apparently this is meant to be another 55nm part with PCI Express 2.0, HDMI, HDCP and UVD. This sounds very unlikely, especially considering that the Radeon HD 2900 XT doesn't have the UVD engine and making this work and then having a new 55nm part taped out in the next couple of months would be something of an engineer feat.
While we're talking rumours, an improved RV630 core is meant to make an appearance as well, called the RV635. This is another 55nm part with PCI Express 2.0, HDMI, HDCP and UVD support.
You can read the Fudzilla article here