If you want to use Quartz Extreme you need to have an AGP card. Otherwise, you might as well keep the card you have. As to the PC versions, there are several PC versions of the Radeon 8500 that you can flash to have work in a PC. But, they are only the 64M versions of the cards, and it doesn't always work, and you can have issues, and can possibly toast the card if done wrong. The PC cards that are most known to work are the non-LE 64M 8500 retail box version, which is no longer made by ATI. You can pick these up for about $110 or so from store online. Or, you can pay $180 and get the Mac version of the 8500 from macmall.com. ATI's has lowered the list price on the 8500 for mac to $199, so if anyone is charging more then that, they are using the old price. So, for about $70-80 more you save the hassel, and possible failure/issues/etc of flashing a PC version.
Brian