I've mentioned this on here before.... it's not in Fairfax, but the Parkway Deli over in Silver Spring is good if you're looking for a Jewish-style deli. They make really good reubens, and probably the best matzoh ball soup I've ever had. They also have kickass breakfast food. You'll also find a pickle bar, which is pretty unique. Prices are reasonable- but beware, the place is always packed. It's definitely worth checking out if you're over that way- it's one of my favorite places in the DC area.
http://www.theparkwaydeli.com/