Trane offers many different models, from good solid consumer grades to really high end 2-stage luxury models. Please try to ask as many questions as possible. There are a lot of variables in this project. Does the coil needs to be replaced? R22 Freon or R401a Puron? Warranty on parts, labor? It's a very good learning experience. IMHO, the comfort and energy savings may not justify the steep price of a 2-stage luxury model.
Once you have the detailed quote from Trane dealer, look up in chinese yellow pages and try to find one who also installs Trane units. Then ask them to beat Trane's quote by $500 or so. If the guy has been in business for many years, and only installs Trane, he might be just as good as Trane's authorized contractor.
YMMV.
