The Inq confirms ATI's R580 will be known as the Radeon X1900 XTX. But they don't know why ATI decided to add an additional X to the name.
The R580 will end up quite close to the clockspeeds of the original Radeon X1800, it will be clocked at 650MHz with 1550MHz memory.
It has three times more Shaders than R520 and in Shader intensive applications should perform quite nice, and we can confirm that it will end up significantly faster than still ghost Geforce 7800 GTX 512 at least in some scenarios. µ
ATI is getting ready to ship them out and they will probably arrive on January 24 or maybe a bit later.