With a few exceptions the 3.3 volt cards won't fit the 1.5 volt AGP slots. The universal cards, dual voltage, will fit.
It's safer to check first. If your motherboard's AGP slot has the double key it should be safe. The 3.3 volt-only cards just won't fit. I don't think there is much to worry about other than Voodoo(x) cards. However, I repeat, better to be sure about the voltages.
<b>I have so many cookies I now have a FAT problem!</b>