yeyordna-- I found that 3rd hostel you're talking about, the HI on Vashon Island... If you plan on having a car and don't mind a $12 ferry fare plus passengers for each time you leave the island, it would be okay. In other words, unless you ONLY plan to visit Vashon Island while you're here, don't bother because it will be prohibitively expensive.
voyd-- there are a bunch of U.S. hostel listings on tpunk!
I didn't see one for Roswell though. Bums are NOT allowed to crash at U.S. hostels; you *must* be from out of town to stay there and the prices are about the same as what you would find in Europe. At least this is the case in Seattle.
A lot of people that apply for jobs with me have just moved here and stay at the hostels til they find a place to live on their own, and almost all of them have told me that the H.I. downtown is better.