Actually 20 minutes off the freeway isn't bad in Boston. Also, depending on your travel patterns you have to be aware that if you're convenient to the Mass Pike (I-90) you won't be convenient to I-93 and vice versa. Getting to I-93 via Storrow Drive, for example, can be highly amusing/irritating depending on how badly you need to get somewhere on time.

You may want to consider East Cambridge (by the Lechmere area) which has relatively decent highway access and is consider an "up and coming" area before the whole housing crash issue. There are some nice condos in the vicinity and amenities aren't bad.

One wild card is the continued bridge renovation projects in the area. The Bourne bridge just started repairs but those are scheduled to be completed by early next year. The Boston University Bridge has been under construction for over a year and won't be complete for another year. This makes traffic on the Cambridge side heading for the pike in the afternoon even worse than it previously had been.

Central Square in Cambridge is where a lot of the homeless/indigent services are located which is why the demographics there after dark is far different than a walk through Davis Square in Somerville at night. For some this can be somewhat disconcerting and Central Square can be somewhat patchwork in terms of safety.

Harvard/Porter are viewed as "nicer" but freeway access is somewhat more difficult from there as well. Never underestimate the time it takes you to drive a mile in Boston during rush hour.

I would not discount Allston/Brighton, particularly since Harvard has pulled back on their expansion plans. This has made rents stabilize to some degree. I think you'll be hard pressed for $1900 overall in Boston but willingness to extend to $2300 should give you some flexibility.

I would check the no-fee listings on the Boston Craigslist to give you an idea of how things are currently on the rental market. January is a good time to look in some ways as the market is quite soft and you'll have some negotiating room as a result. There's less inventory, but the inventory that is available is usually fairly motivated to sign a lease.