ATI today announced the Radeon X800 XT for Mac, at US$499. This card utilizes ATI's latest generation GPU, and competes in performance with the nVidia GeForce 6800 Ultra DDL. The main difference with the GeForce card is that the ATI card sports ADC and DVI instead of dual DVI (although ADC can be converted to DVI with a $30 adapter), and while it can drive a single 30" screen, it can't drive two of them like the GeForce card can. On the other hand, the ATI card is $100 cheaper, and doesn't block a PCI-X slot. One cannot build a G5 at the Apple Store with the X800 XT however, as it is strictly a non-OEM retail product. However, it's possible it will appear with the next G5 Power Mac revisions as an OEM built-to-order product.
The Radeon X800 XT is an AGP Pro 8X card. This means it will only run in a G5 Power Mac, although the good news is that it does not require external power. The bad news is that there's still no evidence that Apple will be releasing a PCI Express Power Mac any time soon.
The first reviews of the X800 XT for Mac are up, at AnandTech and at BareFeats.