If you have the cash to pay for it now might be the time to replace.
Banks etc are not paying out much for CD's or other savings venues.
If you have to finance all bets are off.
A 30 year old unit is using a lot more utilitys than a new one will so you will see a savings there.
Tax credits and rebates may not last forever.
On the other side.
Cash will soon be king and we will see interest rates back at the 1980 level. Our gov cannot keep up without this happening since no one really wants to invest in our debt anymore. I am waiting for the 12% CD's as in the past. LOL
Spending money has never been one of my most cherished things to do.
Whatever you do get at least 2 or 3 estimates before you sign on the dotted line. Below is a good bit of information to take to heart when dealing with your future contractors.