Oops. See you already got a bike....

I got my 20 year old Cannondale R400(criterium bike) out last fall to get back into riding. My back KILLED me. I bought a Specialized Allez. It is kind of an "aggressive" relaxed geometry bike. At first I thought it might be too relaxed but have become very comfortable on the bike. The back pain is gone but then again that Cannondale was known as a "back-killer".
Tell the shop people what you are looking for. Make sure they don't just assume you want an upright bike, that you want something a little aggressive, yet forgiving. They should be able to match you up to the correct bike. Then the fitting should be done pretty meticulously. I had a young guy do mine and he did a fantastic job, that impressed a friend of mine that is very picky about bike set up.