Trip to Europe

My husband and I are in the planning stages of going to a trip to Europe next fall for our 1 year anniversary.  Neither of us has ever been to Europe before, but I have wanted to take a trip there for as long as I can remember.  We are planning on spending between 2 to 3 weeks, probably in October, though it will probably be a little closer to the 2 weeks.  There are so many places I want to visit, so we are having a hard time deciding which cities to go to and how long to spend in each city.  So far, we know we want to go to London, Rome, and possibly Paris (or another city in the South of France).  How long would you suggest staying in London and Rome?  I know there are a lot of neat things to see in Paris, but I’m wondering if we would enjoy a smaller city in the south of France more – any suggestions?  I have also always wanted to go to Spain, so any suggestions on cities to visit in Spain would be greatly appreciated!