Once you have these answers listed above, and review the sites mentioned by each firm, you'll have a good sense of the firms you'll want to bid on your RFP.
Send it out to no more than three firms . You've already done the front work to ensure that the proposals submitted will be serious contenders. If you must, send it out to four firms.
When you receive the proposals, make sure you ask about any content you don't understand.
Begin by evaluating each proposal individually. Evaluate not just what's included in each proposal, but the proposal tone and comprehensiveness. Weigh in on each firm's potential as a long-term partner.
Once that's complete, compare the proposals. How do they fare in terms of presentation? How do their processes appear in terms of project management? Do they present scalability and/or upgrade paths for your project, that go beyond the needs of the goals outlined for the next site?
Before you make a decision, arrange an in-person meeting (if possible) with the finalist firm. If a demo presentation is needed, do not hesitate to ask for one.