Hi, no responses yet so I'll give it a stab. I lived in Metro Washington D.C. (Kensington, MD) and now live in Boston (spent some time near but outside the city too).
Regardless of your budget, you'll get the LEAST bang for your buck in metro New York. You may be able to find a nice home about an hour and a half outside the city along a commuter rail line. Somewhere around New Haven might be nice. New Haven itself has a great downtown area that's seen a LOT of revitalization in recent years. You can also access the Metro North Railroad out of New Haven's State Street or Union Station which is about a 1.5 hour train ride to the Grand Central Terminal. I took a quick peek at craigslist and found
THIS home in Cheshire (a short ride from New Haven) that was within your price range. I know, new, generic "colonial" but it's just a reference point. There are some nice historic neighborhoods in the New Haven area, but the nicer homes will likely cost more than $400,000. Parts of Northern New Jersey and up out past Westchester may offer some similar opportunities. But in order to have a nice home out of the city in New York, you're going to have to have a relatively lengthy commute.
In Washington, you can do a little better. The historic housing stock may be a little more difficult to pinpoint outside the city but give Alexandria a look. Houses will get cheaper as you get out towards Fairfax, VA and outside the inner ring of MD suburbs like Bethesda, Kensington, and Chevy Chase which can be extraordinarily expensive. Washington doesn't have a commuter rail. The "metro" acts as both a subway and a commuter rail line. I find that it's one of the better transit systems in the U.S. and it's pretty extensive.
I think your best bet for finding an established neighborhood (do you mean established as in 1950s style suburban or REALLY established as in early 1900s or late 1800s type neighborhoods?). would exist in some of the exurban areas of Boston. You may REALLY like a town like Newburyport which is loaded with an excellent older housing stock and some great old nieghborhoods. Newburyport has a nice, walkable downtown area and commuter rail access to Boston's North Station. On the South Shore, Plymouth would be a good bet too. Plymouth has some great older neighborhoods as well as a walkable downtown area. It's also on the commuter rail line (to South Station). Towns like Franklin, Lakeville, Westwood, Concord, Rockport etc will also have what your looking for in terms of neighborhoods. I would have suggested Fall River as its Highlands neighborhood would be perfect for you (affordable, gorgeous old homes... ranked the best historic neighborhood in the nation by "This Old House"). Fall River's downfall is its school system (also some rougher spots on the opposite side of the city) but that wouldn't affect you. The problem is that Fall River won't have commuter rail service until 2016 so you'd have to drive to Quincy to pick up the Red Line or Lakeville to get the commuter rail. Plymouth or Newburyport would be my first choices.