These collection types are not exactly interchangeable: NameValueCollection allows access via integer indexes. If you don’t need that functionality, you shouldn’t use a NameValueCollection as indexing doesn’t come “for free”.
Depending on the number of strings you’re looking at, I would consider either Hashtable<string, string> or IDictionary<string, string>. Krzysztof Cwalina discusses the subtleties here: http://blogs.gotdotnet.com/kcwalina/archive/2004/08/06/210297.aspx.