Michael, there's a lot of factors that figure into sizing HVAC units besides square feet. Rules of thumb simply waste energy and cause customers to be uncomfortable. I would suggest that you get a service company to calculate the heat losses through walls, windows, etc., to determine the proper size unit. You should end up with a unit that keeps the house comfortable and saves energy. Also, depending on the existing unit, the duct work might not handle sufficient air flow if you install a larger tonnage blower drive. If you're in a northern state or colder climate, I'd also suggest the highest efficiency furnace that you can afford. If you're in a state where you only use the furnace for a month or so a year, I'd consider the cost of the high efficiency and the payback period. A blower door test would also be benefical. It shows where you're loosing heat and once the areas of loss are corrected, a smaller unit can normally be installed and drafts are eliminated. As to what brand unit? I don't think there's a nickles difference between them. Except for Trane, but that's too expensive for my blood. In closing, the size of the unit will depend on heat loss, climate, insulation, number of windows, etc. One last thought, what does she have now, have you made any changes to the house, is she comfortable with the existing unit, how are the utility bills? Just my thoughts.