actually I have flashed many PC cards with mac firmware 2 of them was ati radeon 9250 with 256mb.
after the flash card could only count 128mb because there weren't 256mb 9200 cards on mac, and also the card is recognized as 9200 and not 9250.
on AGP mac's that came with cards with ADC plug you need to cover pins 3 and 11 on the card thats the 8x setting. Apple used that pins to provide 24volts to cards to power ADC ( there wasn't 8x protocol at that time), so card will work at 4x.
there are 2 ways of doing it
1) put the card on a pc have a DOS bootdisk with atiflash utility and mac rom and flash it from there, then cover pins 3&11 and you are ready to roll.
2) setup a vnc server to your mac, cover pins 3&11 put the card on your mac and boot it (monitor will show nothing because incompatible firmware but mac will boot normally), go to another machine networked on your mac and open a vnc client, use atiflash from aticcelerator package to flash the card, reboot and voila!
Detail instructions for both ways can be found on the above mentioned link at strangedogs.
hint:
mac cards were using 128kb flash rom and most pc cards are using 64kb, so most probable you will need the reduced mac roms that you can find on strangedogs wiki.
the effect of using mac roms is that you will be limited to 1024x768 resolution till driver(kext) loads. (you will need the ati driver from ati site, intergraded drivers from OSX will not work except the fixed 1024x768 resolution)
