Well, we official have CRM 4.0 running on our SBS 2003 r2 server. I had to investigate an issue with SQL Server 2005 during the installation process. There is a hotfix KB936305 that needed to be installed to get SQL Server fully patched. Once we did that the installation worked great...

The outstanding issue that I have relates to the version of Microsoft CRM 4.0 that is allowed to be installed ON the SBS Server. Apparently, after talking with Microsoft Support reps on the private forums, ONLY the Workgroup edition (which is limited to 5 users) is supported on SBS 2003 r2. In my case, the only 'choice' I had when I installed was based on the product key I typed in. I used the Professional 'trial' key, with no problems, but am curious as to how I 'tell' the installation to install the Workgroup (we will have more than 5 users, so we'll need professional----Just like most of my customers).
We already have 1 member server on the domain running our support server, but I'd hate to have to throw a 2nd member server in the mix for maybe 10-15 users of CRM. I'll keep investigating this.
