Lime Kiln and Clarksville Middle are great schools. They're both diverse, have great state test scores, active PTA, etc. Howard County doesn't have a magnet program because each school has GT specialists and provide GT services at all school campuses. Actually, there are plenty of people who commute to DC who live in neighborhoods that are zoned to both of these middle schools. Go to the Howard County School website
http://www.hcpss.org to obtain more information (demographics, test scores, school profiles, etc.) about each school and to look at the school boundary maps. HCPS website and each school's individual website will offer you lots of information.
There are many other great schools. However, with a commute to DC you wouldn't want to go in further north than Highway 32.