The Washington Post published a story the other day that featured this guy:
The article gave advice on how to dress appropriately for outdoor weather when exercising. Some of the advice would also be useful for bike commuting. However, as someone who bike commuted year round for many years, I don't think it's necessary to make the hundreds of dollars investment in expensive gear like this. There's plenty of ways you can meet your needs at reasonable cost. Talk to cyclists you know first.
But whether you want to keep REI in business or not, it's not bad to ride through the winter in the DC area. It gets cold sometimes, but there are many, many days this time of year when the low temperature doesn't even drop below freezing.