There are three voltages used on AGP Video cards: 3.3V, 1.5V, and 0.8V. Early AGP1x/2x cards used 3.3V, the vast majority of AGP2x/4x compatible cards use 1.5V, and AGP8x cards usually support 0.8V, but are backwards compatible to AGP4x with 1.5V.
You do not have to worry about this on modern motherboards and AGP cards - any card made after 1998 will have keying in its AGP connector which will prevent one from installing the wrong card into the wrong slot. Unless you're using a Pentium II motherboard with an AGP8x card, you're going to have a hard time frying an AGP card this way.
In short, the motherboard takes care of this - do not worry about it.