First off to address MSCARD, the Steven F. Udvar-Hazy Center, while part of the Smithsonian Institution, is NOT in the Nation's Capital. The museum sits on Dulles Airport property, entirely inside Fairfax County in the Commonwealth of Virginia. For practical purposes, it has to be on an airport (the former site, the Paul Garber facility was truly inaccessible).
Second, it's not hard to get to. You can take the DC Metro subway to the nearest station (currently Reston-Wiele) and take the bus from there to the door of the museum. In a short while, you'll be able to Metro all the way to the airport. If you are AT the airport already, there is a bus between the te
I'd definitely recommend the visit. Unlike the downtown museum which is a quick in and out, there is lots to see at Hazy. If you're truly interested, walk into the docent desk which is located just to the left side of the SR-71 tail. THey have both regular tours and can talk to you at length about anything you want to know. These guys go through an extensive 20 week set of seminars to learn the basics and have to do continuing education (and most will study more) before they are allowed to volunteer. Many of them are significant aviators in their own right, retired military or airline pilots and the like.