It also might be a good idea to look at randyjawa's site:

http://www.mytenspeeds.com/My_TenSpeeds_1/

It has a section on bicycle quality. There a *lot* of information in there, all of it good, and if you have interest in vintage bikes, then it will tell you what to look for to get a quality frame.

Like I mentioned before, I'm partial to the Japanese mixtes from the early '80s for ease of upgrading and general quality of workmanship and materials. If a steel frame is taken care of, it will last longer than we will, so a good 30-year-old frame may be better in some ways than a modern one.