We guess that ATI will stick with the Radeon HD 3870 name for the RV670XT, while the slower card based on the RV670PRO will be known as the Radeon HD 3850.
The second one, based on the RV670PRO, has 256MB of memory and will be the first one that you will be able to buy. The faster RV670XT iteration will come a bit later, at least as far as retail is concerned. The RV670XT samples are done, but the volume production will focus on the RV670PRO at first.
ATI plans to sell tens of thousands of RV670 based cards and the main focus is the RV670PRO, which is the card many people will sample and have at launch date. The RV670XT will end up faster, a bit more expensive and competitive to Nvidia, but at the same time, there won’t be as many RV670XT cards as RV670PRO. It's a yield thing.