Well, I just took a look at the motherboard's manual, and it says something along the lines of:
1.5(8x/4x) power mode only;
AGP 8x/4x supported
So unless 4x/8x is completely different than 8x/4x, it looks like I should be alright.
EDIT: And after looking at some information on my card's website, it would appear that it runs in 1.5V.
Thanks for your help everyone, I should be good to go.