If you plan to go from an entirely non-PC business to a full client/server networked business environment, you should consider a small pilot project with a few employees. This will help you test the proposition that you will attain many of the efficiency gains you expect and that your employees will be able to handle the change in business practices.
If you plan to migrate from an older client/server or peer-to-peer network, you will want to make sure all your key line-of-business applications work on Small Business Server. Again, you should consider a pilot project in which you install your business application on Small Business Server and test the application to ensure that it operates as you expect it to. Run this application on both the old and the new platforms for a week or until you are comfortable with making the final switch to the new network.