you probably won't be able to find a deal that cash-flows in fairfax county, or in the northern/western end of Prince william county. if you find a good deal you might be able to break even on rent vs PITI, CapEx, maintenance, paying the water bill, and potentially property management.
I have a few friends that have purchased townhouses in the gainesville area and are cash-flow positive because they found a great deal and most importantly because they put a huge amount down for the downpayment or have made substantial payments and pave paid down the principal dramatically.
How soon are you looking to purchase? I know that not a lot of people recommend "waiting to make a purchase", but if you are purchasing in those particular areas, and are purchasing off the MLS, now might not be the best time unless you can find a pretty heavily distressed property. Again, I'm not an expert by any means, but i've lived in this area for a long time and have been keeping a pulse on the market. I have chosen to invest out of state for better cash-flow returns (but I am not experiencing much appreciation where I'm investing)