For processing, Penn's main store is E street, and I think they send everything there. The staff at their stores tends to be very knowledgeable, but often a bit abrupt. You might also want to take a run out to Dominion Camera, which is about a mile from each of the Falls Church stops on the Orange Line. They're a good, old fashioned camera store with labs in house. You can smell the fixer, and the staff are real photogs. Only place I know in town where you can walk in looking for something like a Series filter and not get a blank look. Falls Church is a small town, so there may be things you would like to shoot there too.
BTW, plan on either renting a car or taking some cabs if you are spending time in Georgetown. Metro doesn't go there.