ATI won’t have the usual XTX and the XT alignment. The company decided to change its original plan. At first DAAMIT planned to play the usual game and introduce R600 XTX and R600XT, were the second one is a cheaper and slower clocked versions of the chip.
That won't be the case this time as ATI plans to unify the R600 brand and sell it under one umbrella. The only difference will be in the quantity of memory. The first one out will be GDDR 3 based board with 512 MB of memory codenamed UFO and after it, ATI will release GDDR 4 card with 1024 MB card codenamed Dragonshead2.
One chip only means that the memory will play the difference as well as the memory speed. ATI obviously knows more than it wants to tell the world, and I would bet that the answer is in performance of the R600. You can read more here.