There are two options, one that all the R600 cards will get the HD 2900 brand, while the second claims that some cards might end up with a well known X2900 name. We will vote for the HD one as it simply sounds better.
Anyhow, ATI won't launch an XTX card anytime soon, at least not in May. The launch card is Radeon HD 2900 XT 512 MB GDDR 3 and there won't be an XTX card at the launch. I guess AMD wants to see what Nvidia plans to do and to and follow up with the XTX. The R600 XT DDR 3 was also known as UFO card.
From what we've heard with the latest drivers and around 800 MHz clock the Radeon HD 2900 XT is competitive to Nvidia's G80 GTX, but we are still checking the claims.