the mac edition cards are getting harder and harder to find for the PowerMac's
they had been hard to find already some years ago especially the x800 as mac edition the pc versions you get fairly easy and if you know how to flash them ....
but the 9800 do a good job and might be available somewhere
the geforce 6800 gt or was it gs cant remember had also been good cards , they had dual dvi out great if you have 2 monitors
the others had one dvi and one adc out to my knowledge
but i think all of the above are cards from around 2004
ok there is a geforce 7800 gs definitely flashable
http://www.barefeats.com/mutant3b.html
there are a couple cards mentioned in that article to work with the G5 too so its up to you how good you are at sourcing these cards on ebay or the like or you might even find in some old barn like computershop lying around old stock
but like macforsience said flashing one is not that easy and you might end up with a nice paperweight
but as you dont seem to much bothered about if the card is high performance so look in ebay for G5 graphics card there are always a couple , just use the one with the higher RAM ,
the original 64mb geforce fx5200 ultra are not bad , ok they are not really for gaming or anything demanding ,but you get a picture on the screen (even a moving one if you watch a film ), and i mean they are cheap to get and
a cheap slow but working graphics card is better then a expensive fast one that does not work any more after flashing went wrong