Don't worry about your FSB. You're only concern should be what happens to the AGP bus speed when you change the FSB. Your manual should tell you. If the AGP bus is to high, your card may not work, but I doubt it won't work when you bring the speed back down.. I wouldn't worry about frying it, the only way I can see you doing that is OCing the TNT2U itself with something like PowerStrip. Even then, you would have to really work hard to ruin it.
The more I learn, the more I realize I don't know anything at all.