I get this question quite often: "Is there an upgrade path from Windows Small Business Server 2003?" The question actually comes in many different forms, but essentially, what SBS customers want to know is: What will happen when their business out grows the limitinations built into SBS. For example, what if my Exchange 2003 Server mailbox store grows beyond the 16 gigabyte limit and I want to upgrade to the enterprise edition of Exchange. The same question applies to SQL Server.
The short answer is "YES". There is a way to grow from SBS. To do this, you would purchase the Transition Pack. I did a quick search and found some information on the SBS site on Microsoft.com. Here is a link to that info: http://www.microsoft.com/WindowsServer2003/sbs/techinfo/overview/licensingfaq.mspx#EABAA
The Transition Pack also can be used if you want to move your server apps (SQL, ISA or Exchange) from your SBS Server to a dedicated Windows server.
Hope this helps.