Without some sense of what you could afford (or how small you're willing to go), it's hard to say if it's possible. It will also matter if you want a town with a top school district, etc. For a house with train access to Berklee, I'd look at the towns with trains going through Back Bay station. From there it's a 10-15 minute walk or one stop on the Green Line T to Berklee.
MBTA.com > Commuter Rail Maps and Schedules
Congested is also relative. "Not congested" by Greater Boston standards might be very congested to you. I've spent far too much time in New York City, so there's hardly anything in the Boston area I consider all that congested at this point. The towns west of Boston on the Framingham train line are probably too congested by any measure, until you get out to Ashland and beyond.
I suppose for the best combination of access to Berklee and affordable/not congested, I'd look south at places like Mansfield or Walpole, which are on different train lines and each about 40 mins to Back Bay station. Maybe Canton, which is a little closer.