It's only possible to flash PC cards if there was a Mac version of the card from which the firmware can be used - or a card which used a GPU similar enough. None of the cards you mention have a compatible ROM with which you could flash them. (unless you fancy writing your own firmware and drivers from scratch with no documentation )
The Radeon HD5450, the nVidia GT210 and 8400GS are all Intel generation cards. They all work in hackintoshes but not in PPC generation hardware. I don't think either of the AGP cards work in any Mac full stop.
There are only a couple of cards that straddle PPC/Intel generation machines in any case.