I have a small (23' Sonic masthead sloop) in DC and it is exactly the right size for the Northern Potomac. A larger boat deserves the Chesapeake, which is where we put our newly acquired C30.
If you're in Arlington it is about 45 minutes to Herring Bay via 495 and MD-4. The latter half of it is a very pretty drive and worth the small bit of extra time to put yourself on bigger water. We're in Annandale, which is a comparable distance, and if you haven't settled on a home yet, you may want to investigate it, as we love the area.
Anything within 2 hours of DC is fairly expensive, except for James Creek Marina on the southern tip of DC, nestled between National Defense University and Coast Guard HQ building. Again, I don't recommend keeping a 30 footer on the Potomac, you will quickly get tired of tacking and there is no air in the high summer. Stifling. The freshwater of the upper Potomac also gets pretty crunchy in the winter. Except in actual severe weather, there's no such thing as a bad day sailing on the Chesapeake and MD is way easier with regard to registration/taxes. DC's registration is very light on the fees as well.
Like the others have said, look in and around Deale and Friendship MD for the Chesapeake marinas nearest you. There are plenty, and they all have pros and cons. Herrington Harbour South is well protected, very posh, and much like a country club, but also with the attendant noise of a place that hosts lots of weddings and parties. HH South access to the Bay is a little quicker than the other Herring Bay marinas because you don't have to negotiate the long, shallow bar that runs north/south as shown about halfway up the left hand side on
this chart of Herring Bay. HH North is quieter and is right beside the haulout and dry storage facilities, as well as a whole host of marine service folks. Shipwright and Paradise are just across a tiny channel from HH North, but I don't have any personal experience with them.
Let us know where you decide to keep her and we'll see you on the water.