Our minimum allowable here in TX will be 14 SEER starting in '08. After 16 SEER the prices can get a bit ridiculous, so if it were me...I'd go with the 16 SEER.
Central U.S is a big place, but I'd go with a minimum of a 14 SEER. I'd also go for a unit with R410A refrigerant (some people call this freon) because units with R22 refrigerant can only be made for a few more years (2010). That means R22 refrigerants will start to become in short supply and if you should develop a leak it will be costly to repair.
In CA the min standard is 13 seer. If this is new construction ideally a zone system would work best with two seperate systems. If your are going to drop it into an existing home make sure that the ducts are pressure tested for leaks and that the size of the ducts are sized to the unit(s). the higher the seer the better performce but also the cost goes up with it.
I actually work for an HVAC company. In the past we would sell a 10 SEER however, most of the companies have gotten rid of the 10 SEER because it is not as energy effeciant. You should get a whole bunch of different estimates and pick the brains of the people who come out to you since this is going to be a huge purchase. Chances are you are going to want a 13 SEER and if you can add an energy saver package on it. Although it may cost you an extra grand over the coarse of the 10 years the unit should last you you will save more than that in your energy bills. Also try to go with a company that offers an 10-20 year warantee from the manufacturer on the compressor and/or heat exchanger. Also remember that this is considered a Capital Improvement on your home. Some companies will charge tax to un-knowing people and pocket that extra cash. You should get a certificate of capital improvement to take off on your taxes next year. Good luck.
