Yeah that's a good point and it should be reiterated. Do NOT under any circumstances let a realtor talk you into living in PG County. There are many reasons for this that don't need to be fully explained. Best to simply avoid it. Some parts of Silver Spring are ok, but traffic in Montgomery County is even worse than in VA from what I've seen. Best bet is still Arlington. It will be well worth the extra money you'll likely pay.
i didn't get into this earlier, but true. A group house in arlington (there are tons, again in Ballston, clarendon, courthouse or virginia square)... would be anywhere from 450 - 900... there is virtually no crime, it's a super short ride into the city, and there are lots of grocery stores, biking trails, entertainment, restaurants, etc. either in walking distance or short drive (if walking is important you can screen for that - if you have a car just might open up a few extra options). Once you get out into falls church area the price doesn't fall off that much, but it becomes a lot less convenient.
Not as nice as the areas i referenced, but as cheap or cheaper - Crystal city. on a different metro line, but equally convenient.
I'll put MD like this - people get killed there ALL the time. every night on the news someone is dying in PG county. car jackings. muggings. raping. etc.
Nothing bad seems to ever happen in arlington (esp the neighborhoods I mentioned. again, super easy to get to city (metro, and lots of cabs, or if you have car, a super easy short drive (10 minutes) when there isn't rush hour traffic).