What kind of computer do you have? What do you want to do with it?
VooDoo 1 and 2 cards do not need to be flashed. VooDoo 3 cards do. But I don't know why you would buy these cards; they're out of date and no longer supported. VooDoo 4 and 5 cards cannot be flashed; you must buy the Mac-specific models in order for them to work. Same thing, though; they're no longer supported by anyone and cannot be used in Mac OS X. And they're pretty expensive, too, espcially when compared to a GeForce 2MX. The MX can be bought for a PC and then flashed to a Mac card, although this isn't guaranteed to work (standard disclaimer. I did it and I'm more than happy with it). Otherwise, get a Radeon. More expensive (bout two to three times as expensive as the MX, at least up here in Canada), but guaranteed to work. That does it for decent cards. If you want to go all out, get a GeForce 3. Only get it if you have a Digital Audio or newer G4 though. They DO apparently work on some 400/450/500 models as well, but again, not supported and more likely to not work.