Joe's got great advice - I recently purchased a new 5 ton 19 SEER Lennox with 2 speed compressor, variable speed air handler, and 410a. Expensive, but I plan to stay here a while and in AZ, we need it. I"ve lowered the temp in our house a couple degrees and power bills have gone down from the old single speed 4.5 ton 13 SEER goodman crapola. Can't wait (well, yes, I can - don't like the tab for it) to replace the other 3 ton goodman.
The house is much more even between rooms since the variable speed air handler can be set to "on" all the time, but it's on at a very low speed, but it still helps circulate.
Carrier, Trane, and Lennox are all good brands in my opinion. Goodman is bottom of the barrel.