I got a nice little plastic insert on my p4t-e
Apparently, 3.3V AGP cards dont have the extra 'nook' that 1.5V AGP cards have. So physically, a 3.3V AGP card wont be able to fit in a 1.5V only AGP slot.
However, the only exception are SiS 305 based AGP cards. They are 3.3V only and also have the 'nook' that the 1.5V AGP slot uses. On my manual, it says using such a card will cause 'serious and permanent hardware damage'