Any 4x or higher AGP card can run at 1.5V. I believe some 2x cards could too, but most were 3.3V. So just don't stuff a 2x AGP card in a mobo that doesn't support 3.3V. If I remember right *most* 4x mobos will handle a 3.3V card, but not all, so check. 8x AGP mobos generally only work with 4x and 8x AGP cards.
At any rate, you'll be fine unless your mobo is really old. I have no clue if a GF 6xxx can run on a 1x or 2x board (but why the #&*%^$&^! would you want to?)
You know, it would have made a hell of a lot of sense if they'd keyed the AGP slots for voltage like they do with PCI...