MOBO's that support AGP 4x (max.) will also support 1.5V and 3.3V, with the exception of a few MOBO's that ONLY support 1.5V.
So you should be fine as all AGP 8x cards support 0.8V and 1.5V.
MOBO's that ONLY support 3.3V are AGP 1.0 (1x, 2x AGP) MOBO's.
Also most AGP cards and slots are keyed, so if it isn't compatible it won't fit....there are a few exceptions tho, one of them is the Asus V3800 (I believe) which has a jumper for 3.3V and 1.5V, that card will fit in a 1.5V ONLY MOBO, and could ruin your MOBO if it's set for 3.3V.