If you are in the market for a whole new AC unit I would suggest looking into a heatpump instead of an AC.
I work for an HVAC distributor and heatpumps can save you a lot of money in the fall/spring. They are basically an AC that also works in reverse to pull the warmth out of the air to heat your home. They are what you hear advertised as HybridHeat but not many companies push them because not everyone has them or knows the benefits. That and they use to be only used down south and for commercial jobs.
For example on a typical 1500-2000 sqft home using a 2ton heat pump can save you about $500/year over a 2ton AC. Even more if your electric company offers a dual fuel rate. And you can get some nice rebates from your electrical company and tax breaks on higher end unit and many places offer steep discounts if you also buy a high efficiency furnace too.
:doh: I sound like a saleman... I'm not, but I am admittedly biased as I get profit sharing.