hi,
try updating ure mobo bios, with a beta one if nessissary.
whats probably happened, if its not the AGP8x problem, is that the mobo has tryed to initialise the video card before the extra power can get to all of its components. with an updated bios, it`ll probably delay the video card initialisation a few seconds so that the power can get to all of the video cards components.
if that doesnt work, press the reset button a few seconds after youve booted. this will apply if there isnt a new Bios as well.
as far a i know, this only happens with cards that need an external power connector. (dunno bout voodoo 4&5 though so dont quote me!!!)
hope this helps