The ATI X1900 XT that Apple is offering on thier store will NOT work in a Powermac G5.
However, ATI IS releasing a separate upgrade card just for the G5, which sounds like it's equivilent to an X1900 GT.
"Accelerate your mac" has an article about this and adds some clarification. (The article is approximately in the middle of the page.)
"ATI to release (retail) PCI-Express Radeon X1900 G5 Edition - I called an ATI contact at WWDC to ask if the new "ATI Radeon X1900 XT Graphics Upgrade Kit" at the Apple store (kit priced at $499, est. 3-5 weeks to ship) would work in a PCI-Express based G5 Tower. (The Apple store kit page says it requires a Mac Pro.) I was told the X1900 XT kit card would -not- work in a PCI-e G5.
However he said that they would soon release a "X1900 G5 Edition" for PCI Express G5 towers. (equiv. to the PC X1900 GT card, less pipes than the X1900 XT for the Mac Pro tower).
The X1900 G5 edition was demoed at Siggraph in a PCI-e G5 he said and hopefully will be available at retail in a few weeks. He said the card is compatible with OS X 10.4.7 using the drivers included with card. (No official price yet, but I think it will be significantly less than the X1900 XT's Kit price.)
BTW - a Quad G5 owner with OEM Nvidia 7800GT card asked if it would work in a new Mac Pro tower. Nobody I know has a Mac Pro yet, but if anyone gets one and/or can comment on this let me know. (Update - one reader said it would not work, as Mac Pro graphics cards must be EFI compliant.)"