If you are going to replace the motherboard, because it is an old computer, you might as well just replace the whole thing with a cheap user computer (from some place like
http://www.pacificgeek.com/), or a new model.
Messing around with a motherboard on an old computer is really a recipe for wasting a lot of money. Many of those computers are available on pacificgeek for less than $100 (if you sign up for the newsletter, they often have free shipping deals as well) and honestly you would be lucky to come away with a bill for less than $100 on a project like this, even if the motherboard is the only problem.