Hmm... I haven't done AC work in several years so take what I write with that in mind.
First off where do you live?
After that what is the R rating for the insulation in your attic space and walls? How many windows that are on a predominantly sunny side of the house? Ceiling height (8 feet? 10 feet? Peaked roof?) Where are the current registers located (ceiling or floor), do the registers point at the walls or towards the center of the house? What's the ductwork in the attic made of (pipe or duct board)? Is the ductwork in good condition?
I know it's a lot of questions but they are the kinds of things that need to be answered before you upgrade to a new system. As examples only my last house in New Mexico had was 2000 square feet, two stories with a cathedral ceiling in the living area. and we cooled it with a 2 ton unit. Great insulation and large shade trees helped a lot. The house before that was here in Texas and when it came time to replace the failing 4 ton monster outside I bought a used 3 ton unit that was newer and with the money I saved from not buying a new one I upgraded the attic insulation and cut in a peak vent. The house cooled wonderfully after that. The house we just had built (also here in Texas) is 2200 square feet and has a 3 ton unit on it. Insulated windows and upgraded insulation in the floors/walls/ceiling were cheaper than a bigger unit.
Sorry about not giving you an easy answer. If it was me adding on to our house I'd have the ductwork in the attic redone to cover the entire house, upgrade to new registers that have curved blades, make sure the registers point towards the exterior walls (largest heat load) and make sure the coil in the air handler is clean and clear. I'd also cut in a roof peak vent so that all the heat in the attic has a place to easily escape. I'd also upgrade the attic insulation as much as I could reasonably afford.
Hope that helps.
RichnSteph
