Carolyn,
If you want to use SQL Server to store your data and use Access as the front end to query and create reports from a client work station, you will need a licensed copy of Access on each client work station as well as a licensed copy of SQL Server on the server.
First you will need to determin which edition of SQL Server will meet your business needs (http://www.microsoft.com/sql/editions/default.mspx). You will need an Enterprise, Standard, or Workgroup Edtion. The Express and Developer Edition will probably not be appropriate. Then you will need to pick a licensing option (http://www.microsoft.com/sql/howtobuy/default.mspx).
At my last employer, I was charged with a similar project. I was to change the datastore to SQL Server, but retain all the existing queries and reports in the various Access databases. We had about 200 networked PC's, about 20 Access users and we used SQL Server Standard Edition with a Per-Processor license.
Regards,
Jon