Appreciate all the responses. I live in St. Louis, MO area, and my furnace is down in an unfinished basement, and the a/c is outside. So I don't have the attic heat issue to deal with where I am.
So it sounds like the sooner the better, or at least when I don't have a broken unit that I am desperate to get fixed.
Absolutely. Anything mechanical can break at any given time. Have you had your system serviced regularly? I'd look for a reputable contractor to service your existing 17 yr old system and to give you an opinion on it's condition. It might be in great shape...can't tell from here. At least have it checked....if it's on it's last legs they can tell you. They can also tell you how you can benefit from a new system with a greater effeciency.
If everything was always done "by the book"....the book would never change.