Video cards require their firmware to be written for the OS and hardware they are used with, so you need to get a card that's specific to your computer. It's somewhat common for vendors to make one card and two firmwares for it, and sell them under slightly different model names. Often these different models have very different prices, and I've seen several examples of people buying a card for windows, flashing it to mac, and saving themselves some money because the windows card cost less than the mac card.
If you want to do this you will need to find information on how to do it with the specific card you have in mind. There's probably a few places online that discuss this specific task, but I don't know where you'd find them. Not every card comes in both mac and windows flavors, and not every card that does can be reflashed to the other platform. (usually requires installing the card into a windows computer, obtaining a firmware updater for windows, downloading the mac firmware file, hacking the updater to accept the mac firmware, and flashing it - sometimes it's not necessary to hack the installer) It's about on par with jailbreaking an iphone.
