Yeah, I actually live in Crystal City. You take the Yellow Line from Crystal City to L'Enfant Plaza, then you switch to the Green Line and take it two stops.
EDIT: one last plug. If you stay in Crystal City, make sure to visit Kabob Palace, a 24-hour Afghan restaurant. The food is amazing and I doubt there is anything else like it in the US. It should be within easy walking distance of any hotel in the neighborhood. Yelp link: http://www.yelp.com/biz/kabob-palace-arlington-2
(ignore the 11 PM closing time listed on Yelp: this place is open 24 hours a day, every day)