So what should we be doing with those names? Probably 90% of those are people I don't know." 400 names is a great start for a mailing list. Certainly mailing list you want to grow your list, but be careful of adding just any names. It's best to add the names of people who will make good donors. Typically, the chamber list or similar lists aren't going to be very productive.
Be sure to monitor it closely to see how it does and stop using it if you don't see the results you want. Start by separating the group into mailing list two lists. If your first 400 names are current donors or people who have expressed interest, consider them your "house" list. The new people are "acquired" names. People on your house list have expressed their interest in your organization by making a gift or have said they care about your work. Be sure to communicate with them regularly.
I'd probably send at least 2 newsletters and 2 direct mail appeals across the next 12 months. This will help keep your organization at the top of their minds. New people you have added won't know much about your organization, so send mailing list them a letter telling them about your organization and how they can get involved. Be sure to mark these names in your database somehow so you know where you got them.