>If I am looking to upsize my application to a SQL back end, which edition do I need and which edition can I distribute to clients?
>
>I have an MSDN subscription so I can install everything from Express to Enterprise, but I assume I should test with what my client will use? can both editions sit side by side?
>
>Gary.

Yes, you can install several editions on the same server (and several versions). Depending on the budget most likely the customers will go with the Express or Standard version, but I think you may want to install Developer's version for your tests and Enterprise version on the Server.

If you want to include distribution of the SQL Server to your clients (installing it), then you can only do this with the Express version (free), as far as I know.