Ok, this is what I would get for a $100:
GIGABYTE "GA-K8NF-9" NVIDIA nForce4 4X Chipset Motherboard For AMD Socket 939 CPU
It's exactly the one I got. It's hard to find the mobo of your specs from Newegg without going over the $100 mark, unless you go with a cheap manufactured motherboard. Gigabyte is proven reliable with me so I am recommending this to you. It's about $20 over your budget, but it's a good board that supports your Athlon 64 and has PCI-e interface.