If this is going to be during the week, I would strongly suggest getting a hotel near IAD instead of DCA. The Beltway is its own circle of hell at morning rush hour, which starts at like 6 and probably doesn't end until 10 (in other words, right when you are going to want to be headed towards IAD if you overnight in Crystal City as has been previously suggested). By the time you arrive at DCA, on the other hand, the evening rush should be largely over and the journey to an IAD-area airport hotel smoother.