I tried searching for posts about this before I posted, but I couldn't find any.

I just got new 80/100 Watt bulbs (Matrix Ultra White) to replace my yellowish Sylvania headlight bulbs. I've read somewhere about having to upgrade the wiring harnesses or the high wattage bulbs will melt them. Do I have to upgrade my wiring harnesses? If so, how much will it set me back?

Yes, you do have to upgrade the harness or you will burn up those wires and then you won't have any lights.

I believe they cost around $20-40, you can get them from a number of places.

-C-