Flashing isn't hard, but it's definitely a major risk - any mistakes or incompatibilities will render the card virtually 100% dead.
Some cards are easier than others. The Voodoo series (at least the 3000/4000) were designed to be flashable between Mac and PC. Some retail cards from ATI can be flashed without a problem, but not all of them (mostly the RADEON 7000/8000 series and some older RAGE cards). A few retail Matrox cards and some nVidia examples can also be flashed.
However, often OE cards will have differences that keep them from being flashed, so they're not usually good choices. A good example of these would be some of the OE Adaptec PCI UW SCSI cards. While they make a Mac version of these cards (the PowerDomain), the PC OEM versions have a smaller flash chip that's incapable of storing the code for Macs. Therefore, when someone tries to flash one of these cards to work on a Mac, it results in a dead card.
This is one of the situations where you'd really want to utilize Google to its fullest extent. This here page may give you a hint as to which cards would work best. You should also spend as little money as possible on cards to be flashed (except the Voodoo 3000/4000), because if the card gets hosed, you're almost completely out of luck.
Some cards are easier than others. The Voodoo series (at least the 3000/4000) were designed to be flashable between Mac and PC. Some retail cards from ATI can be flashed without a problem, but not all of them (mostly the RADEON 7000/8000 series and some older RAGE cards). A few retail Matrox cards and some nVidia examples can also be flashed.
However, often OE cards will have differences that keep them from being flashed, so they're not usually good choices. A good example of these would be some of the OE Adaptec PCI UW SCSI cards. While they make a Mac version of these cards (the PowerDomain), the PC OEM versions have a smaller flash chip that's incapable of storing the code for Macs. Therefore, when someone tries to flash one of these cards to work on a Mac, it results in a dead card.
This is one of the situations where you'd really want to utilize Google to its fullest extent. This here page may give you a hint as to which cards would work best. You should also spend as little money as possible on cards to be flashed (except the Voodoo 3000/4000), because if the card gets hosed, you're almost completely out of luck.