Please state the model you have. You can check the motherboard's manual to see whether or not the board will support a 3.3V card, however, chances are it won't. Also, some 4x AGP cards happen to operate only as 3.3V devices, namely older RIVA TNT2-based cards.
You can usually tell whether a 3.3V card won't work is that the card will not fit in properly into the slot (though with the exceptions of some 3.3 cards)
An AGP 4x/2x card will work in it, as long as it is a 1.5V device.