Is the above your board?
Asus P4P800S-X Socket 478 ATX Mobo
If so the specs state: Expansion Slots 1 x AGP 8X, (0.8V, 1.5V only)
As in "Only 0.8V & 1.5V"
The 8X will run off the 0.8V
& that CPU should be fine too.
http://www.directron.com/p4p800sx.html