Iâ€™ve been answering questions about the anticipated Green Line extension for a long time. Attached is a blog entry from 2009. My prediction then was not too far off.
Another thing is that the Red Line is currently the only subway line in town, serving a small percentage of the city of Somerville. The Green line will add to the pool of available housing that is near a subway line. That makes property on all the subway lines not be as unique (and valuable) as when there was only the Red Line and the properties near it. (Demand gets diluted when there is more supply.) This could be a factor by 2020 or so.
My general advice on purchasing is to buy what you will love to live in for the next 10 years. If you see appreciation before then, you are lucky. If you don't, you still have a great place to live. If the passing trains are something you won't like in the future, you may want to look for a nicely renovated space a short walk from a proposed Green Line stop.
I think a station nearby can only increase your property value.
However, for Somerville to become "the new Brookline" the city will need to invest in improving the reputation of the school system. From what I see, people flock to Brookline for the schools first, and the transportation second.