Didn't see this thread earlier, mostly because I lurk on Varsity. Hopefully some of you who posted here will see it and maybe it will give you some ideas of where to try Mexican here in Northern VA.
There are several good Mexican restaurants in DC. Chuys is in Fairfax City and in Springfield Mall. Uncle Julios in Woodbridge is good, much better than the one in Fairfax Corner. Anita's can also be good, especially if you like New Mexico green chili sauce. There are many around -- Burke, Vienna, Fairfax.
I've eaten at Guapos in several locations (Fairfax, Shirlington, Rockville/Germantown), and Austin Grill in two locations (Springfield, Fairfax City - before it closed) and I would say those are "Tier 2" options. If you're a little more adventurous, DC is home to one of the largest concentrations of central american populations in the US. There are any number of Salvadoran options in most DC suburbs and exurbs.
And to the OP, we welcome you with open arms. We can definitely use more authentic TexMex options in these parts.