The Inquirer posted some information on the ATI R580 (Radeon X1900) architecture. They reveal the Radeon X1900 XTX will be clocked at 650MHz with 1550MHz memory.
One of the strange things they report is that you'll need a Radeon X1900 XT CrossFire edition, which is clocked at 625MHz core with 1450MHz memory, to make a pair. The Radeon X1900 XT is also clocked at 625MHz core and 1450MHz memory.
The R580 has 48 pixel processors, or should we say pixel shaders, while it still has only sixteen pipelines. We don’t care as long as it gives a high score. The card has eight vertex shaders but you don’t need many of those anyway. It is now the fight of shader information versus pure pixels, as Nvidia can process more pixels per clock while ATI can process more pixel information per clock. There is no right or wrong in this case as it will depend on the game you are playing.
ATI's R580 uses its ring bus memory controller that has a sort of 512 bit internal memory controller. It uses 256 bit 8 channel GDDR 3 and it can support GDDR 4 as well. It is, of course, a native PCIe card and it supports dynamic voltage and clock speed control. So overclocking should not be that hard.