Well, it's always a gamble with the 495. We were driving on it around 10AM today and were stuck in traffic for a while because of an overturned vehicle that shutdown three lanes. It seemed like there were accidents happening all day long today on 495 and 95S that were causing all sorts of congestion problems. This is when GPS and/or old-fashioned maps and finding alternate routes comes in very handy. As soon as you can get it listen to 103.5 FM WTOP. They give traffic reports for the DC area every 10 minutes on the 8's (i.e. 1:08, 1:18, 1:28, etc). That'll at least let you know if there's anything major happening on 495/95S/395.
In your case it might not be so bad since you'll probably be getting to Alexandria around 8-9PM. However, I'm not sure what the roads between NY and VA are going to be like. The tolls might be clogged up still around NY, NJ and DE.