ATI's R580 will be called Radeon X1900 XTX and The Inq today heard at CES 2006 why ATI decided to add the additional x.
It's because the XTX cards will be available from day one, contrary to previous launches of XT cards.
ATI's Radeon X1800 XT was not available immediately from launch but it has now been in the shops since early November. But the chip firm assures us that the XTX will be available from day one, and that it's even better, cooler and faster than the R520. It also appears that the volume of R580 production is better than R520's.