All of the flashing of PC video cards to work in your Mac, as far as I know, use nVidia cards - specifically the original GeForce3, not the current Ti series. I flashed a VisionTek GeForce3 months ago and it works great.
aTi's are not flashable is seems.
There are a couple of very long threads over at the <a href="http://bbs.xlr8yourmac.com/cgi-bin/Ultimate.cgi
" target="_blank">Xlr8YourMac Forums</a>, but unfortunately they seem to be down for the past couple of days.
When it's back up, <a href="http://bbs.xlr8yourmac.com/ubb/Forum2/HTML/001342.html
" target="_blank">check this link regarding flashing</a>.