Hi steinflg,
As the saying goes … there’s no “bad” questions except the ones that don’t get asked 
The first thing I would do is read these guidelines. They will help you with the most important first step which is the importance of identifying the better outlets in your area. In most cases these are either factory direct manufacturers or sleep shops that carry alternative or local brands. Both of these tend to have the knowledge and ability to help “educate” you about mattress materials and construction and will tend to sell mattresses that have much better quality and value. They will also help you make much better and higher quality choices inside your budget. The more they know … the less you have to know … and chain stores and the types of outlets that tend to focus on major brands usually sell “stories” more than they provide good information that makes comparisons between brands and mattresses more meaningful.
If you want to do a little bit of preliminary reading about mattress materials, different types of construction, and how to “fit” a mattress to your needs and preferences, half an hour or so spent reading the overviews in the mattresses section will certainly give you enough information that you will be better informed than many people who sell mattresses.
While there aren’t a lot of great choices in the overall DC region, there are some that are better than others. Post #2 here has a list of some of the better possibilities.
A few others that may be worth adding to your options are …
http://www.baersmattressden.com/ Retailer with outlets in Leesburg, Ranson, and Frederick. Carry Therapedic which makes some gel memory foam mattresses which are good quality and may have better value (if they carry them).
http://hardwoodartisans.com/contactus.html Retailer with locations in Rockville, MD. and Fairfax, Shirlington, Woodbridge, and Elkwood, VA. Carries the Pure Latex Bliss Talalay latex mattresses. Call first because different stores carry different models. they also carry White Lotus futons.
http://www.wolffurniture.com/current/stores/wolf_furniture_stores.aspx is a retail outlet with stores in Frederick and Leesburg that carries Restonic which may also have some mattresses that have better value and may be worth including in your research.
http://www.mtairymattress.com/ Retailer in Mt. Airy, MD. Carries Restonic which makes some mattresses including gel memory foam which may have better value.
http://mattressfame.com/ Retailer in Rockville, MD. They carry several brands which may have better materials, quality, and value including Therapedic, Eclipse, and Eastman House, Englander, Pure latex bliss. They carry latex, latex hybrids and other options such as gel memory foam that may have some better value and are open to supplying the specs of their mattresses (to the degree that the information from their manufacturers allow it) so customers can make more meaningful comparisons.
http://www.abbafutons.com/ Gaithersburg, MD Foam and Upholstery shop that also sells some mattresses and futons. Talked with him and he knows his foam. Carries poly and memory foam but no latex. Sells Otis futons and basic foam mattresses.
http://magicmattress.com/ Westminster, Eldersburg, MD. Paramount, Restonic, and Symbol. Long term retailer with good factory connections and said he would get foam specs if asked (and make sure you ask).
http://americanfoamcenter.com/ Hagerstown, MD., Fairfax, VA. They are a foam outlet but are also a local factory direct mattress manufacturer. I talked with Robert (who is the owner) and it’s clear that he “knows his stuff” about foam. He has been making mattresses for 50 years and is the kind of person that will give you accurate information. He also has a “showroom” (of sorts) where different materials and layers can be tested. He uses high quality materials like Latex (Dunlop and Talalay), high quality polyfoam up to 2.8 lbs (true HR foam) and memory foam from a quality American supplier. Some good quality and better value choices here compared to most mainstream choices and they would certainly be worth including in your research. Make sure you call first because Robert is the person there with the knowledge.
I would also strongly suggest that you first talk to any outlet you are considering on the phone to get a sense of their level of knowledge and service (how willing they are to talk with you about their mattresses on the phone without insisting that you first go there before they will answer any questions) and to make sure they will give you the specifics of the layers of their mattresses (specifically the density of any memory foam and polyfoam in their mattresses).
If you have questions along the way … feel free to post them.
Phoenix