Probably something along the lines of ASUS P4P800SE Socket 478 Intel 865PE ATX Intel Motherboard would be under $100 and suit you well.

On second thought you might need a 775 socket motherboard instead of 478.

Of course you could go with SiS chipset and save quite a bit, but 865 on Asus has been extremely stable for me.

Just stick with a good solid manufacturer and you'll be alright.