If it is flashed for a mac then yes but if not and want to do it very cheaply then you need a pc to flash the card then it will work. Why not aim higher like a x1900xt which is a Lot faster than a x1650. At least I know a few things about macs and own a ibook with ssd mod.
You know what ask around on some Mac forums seeking advice from people who have either worked with or owned a late model G5 as they can give better advice. If money isn't to limited and support under osx for ppc machines that you could look at a mac flashed 8800gt. That will blow away your current card. If not then look at a x1900xt. Yes they are noisy and put out some heat but easy to take care of. To clean it all you need is a brush lol at the in take at the end of the card.