Yes, if you are lucky and have time plus needed hardware for flashing the ROM.
(Notice, X1900 does not have hardware acceleration in 10.6, if you care. At least until someone finds a solution.)
P. S. I think one US vendor on eBay still sells refurbished Mac versions of this card. I got mine from him. (I do not want to post links, but it he still got the stock, it should be easy to find.)
Yes, any of the Radeon X1900 GT, X1900 XT and X1950 XT can be flashed to work in a PCIe Power Mac G5. See the link quoted above for detailed instructions.
Note that on any of these cards, only 256 MB of VRAM will be recognized. Also any X1900 XTX or X1950 XTX will NOT work (at least no one seems to have figured it out as of yet!).