I won't repeat the answers to your questions from other agents. However, I specialize in multifamily investments and am familiar with the Somerville market that you are interested in. I think my piece on investing in the two-family Medford market would be interesting to you: http://www.richleere.com/2010/11/24/medford-mas-2-unit-multi I can get you the equivalent numbers for the Somerville market.
Investing in a two family is a very wise choice. There are many factors to consider when making the decision to buy a multifamily of any type. You want to consider how easy it will be to find good tenants. You want to consider which property will give you the best return on your investment. You want to consider what type of lifestyle you and perhaps your family want. I can help you out with these questions and more.