Answers

GAVIGAN HOMES// Go 15 Seer minimum and check your duct work. Metal boxes instead of fiber board are huge mold mitigation upgrades. Get a blower door test 

I am assuming your home is ducted and has a forced air system.  If you you only have 1 furnace and 1 condenser, it my be more energy effecient to expend to two units.  I recommend Lennox.  You can also look at installing spilt units like these made by Fujitsu:

http://www.fujitsugeneral.com/products.htm

These are designed so that you can cool a room/rooms individually.  I suggest calling a few licensed HVAC contractors and get 3 quotes.