If you have not already used the energy home improvement tax credit, there are still some tax credits available for hvac replacements. If you can use it, then verify that your system will qualify. You should ask the dealer for the AHRI # of the system matchup, and then verify that on the AHRI website.

Also, often some utilities offer particular credits or rate reductions for systems that meet particular efficiencies. So you may want to check with your gas and electric companies about that, and again make sure what you get will apply.

It is the efficiency of the matchup of the outdoor unit, indoor unit, and coil that gives the overall system efficiency and AHRI number. Also some furnaces and coils have their own matchups.

For more info see the heating and ac forums.