ya got 20 years from the old reem/ruud, a new one sound wise to me. however, if it's running, but not cooling, a quality service call could get you another 4 years from the system, and allow you to save up for the premium system. it's amazing what cleaning, and sealing up an older a/c will do.
you may also have building envelope problems that have increased the load on your house itself... you may want to consult an energy audit to evaluate the home. cracks in the ceiling, along with poor attic ventilation will MASSIVELY affect the cooling abilities of any a/c system.
The TRUE highest cost system is the system not installed properly...
The three big summer hearththrobs...
The A/C repairman