Help with SQL Server licensing

The cheapest (free) solution is the Express edition, if you can get along with the limitations:

   http://www.microsoft.com/sqlserver/2005/en/us/express.aspx

Using is allowed regardless of the commercial status of your organization, so it doesn't matter if you are a "non-profit".

I am checking to see if the application will support the express version.

The SQL server will be standalone.

 I see that SQL 2008 R2 can can handle 10GB of data, but uses only 1GB of memory. I assume that this means that this version will be very much slower than the next highest (Workgroup) which can use 4 GB? I have an x64 server. Will there be any issue with express?

Is there any issue in express of users licenses?

In a more advanced version, how is licensing handled in a thin client environment?

Upping points. Looks like I will need all the help I can get here.

I am informed that I need the Standard version. So the question remains as to how licensing works with a Terminal Server.

"Server plus Device CALs.  Under this model, a server license is required for each
operating system environment running an instance of SQL Server"

Does this mean that for each thin client session on the Terminal Server running the software will need a device license?