> they [...] appear to not work in G5’s without a reflash.
The only X19xx series card that's also been available with a PPC ROM is the X1900GT.
PPCrom-flashed or X1900 G5 Mac Edition are great cards for MorphOS and OSX, and also Openfirmware.
They are only single-slot and have TDP about 75W. I am using this card with my Quad and I am satisfied. Only limitation is, that these cards with PPC ROM doesn't work with linux ( framebuffer only, no hw accel ). If you are decide to use linux, you have to use second card like HD6670.X1950 XTX
is more powerful, probably great for MorphOS and linux (I don't test it), but doesn't work with OSX and Openfirmware and it is dual-slot with TDP 125W.
I that case you also need second card which works with Openfirmware (and OSX if you want) - for example original PPC GeForce 6600. Flash mention this combination in comment #9
AmigaOS: Micro A1-C, AmigaOne XE, Pegasos II, Sam440ep, Sam440ep-flex, AmigaOneX1000
MorphOS: Efika 5200b, Pegasos I, Pegasos II, Powerbook G4, Mac Mini, iMac G5, Powermac G5 Quad