The strangedogs people have been flashing 7800GTs for $450, but last I checked, they had run into issues with the latest 7800 core revision. It might be awhile before they get the firmware adjusted enough to work around that.
ATi is coming out with a retail X1900 for the PCIe G5s, but I have not heard when it will come out. Considering ATi's previous retail prices, I'd expect the price to be above $400, and probably $499.
XFX let it
out in March that they were developing nVidia retail cards for PowerMacs. There has been no word since.
As Leonard says, the original 7800GT can be ordered as a replacement part, but it will be very expensive that way.