Here is a link to a consumer level checklist to help you evaluate any proposals you request for replacing or improving your heating and cooling: Quality Installation Checklist. Use it to find the best contractor and value for your investment.
As part of my day job I spent the day in training for correctly sizing HVAC Systems. Here is a link to the event: http://hvacloadslewisandclark.eventbrite.com/
I have been involved and aware of these processes at a high level for many years but now I'm drilling down into some of the nitty gritty details of determining the correct load, the right-sized system and properly designed duct work to maximize comfort and efficiency. A lot of this has technical aspects that are boring beyond belief, but they yield real value.