The Reston Town Center is a good place to live if you're younger and single. It's in the middle of the suburbs, but it's self-contained and has a lot of bars, restaurants, and shops. It would also make for a pretty decent commute for work, as it would be against the primary flow of "rush hour" traffic, and you should be able to avoid the tollway (the main artery connecting those two points). I'm not sure about the $1,200 limit though - that's not a lot for this area.
Other than that... You're not going to want to live in Sterling or Dulles. Sterling has a lot of "bad" (higher crime) areas, and in general they're both really designed for families who want to get out, shop, eat, and go home before 9pm. Not to say there aren't some bars scattered around, but it'd require getting out and driving.
Farther out than that, I'd probably look at the Ballston/Clarendon/Court House areas, which are along the metro route in Arlington. These are supposed to be good areas for singles - meaning there are places to go eat and drink that don't require driving, and there's good access to the metro so you can get far into DC for access to more singles and nightlife if you want to.
Again, the $1,200 might be a bit of a constraint - at least in terms of quality. You can find somewhere to live, but you might not be living the "big-balling, single guy, lifestyle" you imagine, or that you could have for that money in other areas.
