Hi Matt,
Recently I looked into broadband but, because of the remote location where I live, there was nothing available.
DECO, the equivalent of Consumer's Union in the US, has a magazine Pro Teste, the equivalent of Consumer Reports over there, has a very good article "Navegar com limites" in Vol. 261, Sept 2005 on broadband. The two best companies were Clix (www.clix.pt) and Oni (www.oninet.pt) and, although I didn't go into much detail when I learned that they don't operate near me, I have the impression that you can get them for about €25/month. They were much lower than any others. Since you live in a much more built up area than I, very probably your phone will be in one of their Zones.
You don't need to know much Portuguese to read the article. It is in tabular form and about 20-30 Portuguese words should give you the flavour of it.
Good reading 
Val