When you say replacement windows you do mean that the windows will be the exact same size and you are going to be using the existing casing and framing that is already there correct?
When looking at windows the main things you want to look for double or even triple paned, low e glass, argon gas and NFRC rating (the lower the better). Also decide what king you want (vinyl, wood) Vinyl would be your best bet for virtually maintenance free.
what replacement window do you recommend? are the pella themastar good or not?
thermastar is pellas low end window, i looked at a couple of their sliding doors until I found out they didn't meet the specs for the 30% federal tax credit, decided to go with Anderson and I have no regrets.