If you are not having all of your ductwork replaced, which is at least 19 years old based on your post and doesn't meet today's standards, then I would not pay the extra for the 19 Seer system. You would be spending the money for the efficiency and then losing the gain in efficiency out through old ductwork that will have leaks and will not be insulated as well.

Thanks for the info. They mentioned doing some minor work to improve the venting and bring the outside venting up to current code. I've never heard of replacing ductwork. Do you actually mean removing drywall to tear out the old ductwork and replace it with new ductwork? Could you send me a link that explains this in more detail. Thanks...

I don't have a link that I can share, sorry. Most ductwork runs under the house in a crawl space or in the attic above the ceiling. If the ductwork was metal, then it will not be sealed at all the seams/joints according to today standards. If it was sealed, the sealant could have dried up and cracked over the years. If it is flexible ducts, they can split due to exposure to heat and moisture over the years. If they are taped together, then the tape dries up and cracks off over time. They are probably not insulated to today's standards. If you have cracks in joints, inefficient ducts due to not enough insulation or splitting ducts then it becomes a waste of money.

I appreciate your advise. You gave me a different and very useful perspective on things. Luckily, we don't have any flexible ducts; its all metal. There is some ductwork in the attic and the local electric utility sealed this about 21 years ago. I don't know what it was sealed with but I vaguely remember the person used a backpack-like machine with a hose and the word mastic was used. That ductwork is now covered with a couple of feet of insulation so I'd rather not disturb it. There is some exposed ductwork in the basement and the joints were sealed with mastic metal tape five years ago. Its not insulated, yet it could be. But that still leaves the majority of ductwork that's in the walls that wouldn't be sealed or insulated to today's standards.

By now i am sure you are enjoying your new HVAC system, insulating duct work and making sure air leaks are minimum will diffidently help. I would insulate the little exposed duct work in the basement as well. this is a tool used by most HVAC Contractors to calculate energy savings vs SEER and EER.

KEEP AN EYE ON YOUR AIR FILTER

http://www.lennox.com/buyers-guide/tools/energy-savings-calculator