I know you really hadn't planned on it, but with a halfway decent case costing between $25-30, why don't you consider buying her a new one? Besides, if you get a new motherboard, cpu, ram etc.............you run the risk that the power supply you have now isn't good enough. Then if you go buy a new one , an ATX power supply may not fit.
I ran into a similar problem trying to upgrade my girlfriends "Gateway". I put new parts in it, but didn't consider she may need a new power supply. I got a new one, and it wouldn't fit in the case.
If i were you, and you're going to put all those new parts in, I'd get a cheapo case ( it's your mom, how good does it have to be , right

. That way you can keep it for future upgrades. Besides, why waste time looking for an AT motherboard, when a good enough ATX motherboard probably costs about the same.