Jack has some good advise. I have used the heat gain/loss calculating software available at: http://www.hvaccalc.com/ and had very good results. Homeowner single use license in around $34 and it is very easy to use. I have found it difficult to find a contractor that will go to the trouble of doing the calculations, or if they do, they can charge a lot. By doing it yourself in 1-2 hours you can then hand a copy of the load calcs to each contractor and get apples to apples price quotes from them. I have no affiliation with this software, it has done well by me and I whole-heartedly recommend it for any homeowner. It can also easily serve to help you understand exactly what you might be able to change in the house to make it more efficient. This is because it can give you incremental and real time load calcs. For instance you can quickly change the attic insulation value from 6" to 12" and immediately see how that affects the overall heat gain/loss of the house. Pretty neat!