ATI didn't have many more numbers left and it wanted to keep the XTX and XT sub-brands alive. And it figured that X1990 would be too complicated.The clock speeds aren't know yet but it's known now that these cards will feature GDDR4 memory. The release is slated for September/October.
This is rather evolution than a revolution, just slightly faster and that's it. The faster one, Radeon X1950XTX is set to replace the current-fastest ATI Radeon X1900 XTX while the new Radeon X1950XT aims to replace X1900XT cards.
Source: The Inq.