:Generally, the notch of 1.5V AGP slot is different from 3.3V AGP slot. Please refer to the following picture. If your AGP card has the 1.5V key, it is able to work on a 1.5V AGP slot. But few AGP adapters such as SiS305 based cards do not match the rule.
Please check detail AGP SPEC form http://www.ertyu.org/~steven_nikkel/agpcompatibility.html
If you are not sure what your AGP card is 1.5V or 3.3V, please double check with the card vender.