You are in a great price range to consider FHA. Are you checking that out? You can get in with only 3% down!!! And, you can ask for the seller to pay closing costs, etc. Outside of FHA, I am not sure you can have the seller pay toward your down payment (and with FHA it is seller funded down payment assistance which recent legislation is taking away - it's still available if you close by 9/30).
In Arlington, it is TOUGH to find single family homes available at $400K. However, I am pleased to tell you I do have an listing that the price was dropped TODAY to $400K, and I'd be happy to talk with you about it. (see link).
Anyway - other than that brief commercial, my point is that generally homes priced that well go quickly, we can see multiple offers and sometimes there is not the negotiation room that you hear about on TV. It depends on if the property is overpriced. Stuff that's priced well is selling quickly. Now that my listing is priced well, I hope it will sell quickly, too! :-)