> Apple didn't make X850XT cards

Apple also didn't make X1900GT cards, yet they do exist for the PCIe-based PowerMac G5, and were made by ATI :-)
And as you know, while not available in a retail version, Apple actually did offer the X850XT as BTO option with the early 2005 PowerMac7,3:

http://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=11463&forum=11&start=6

> there is one higher card that will work with the G5 as well,
> a standard PC X1950Pro AGP card.

Wouldn't this also be true for other cards from X1xxx series?