Personally speaking, I wouldn't want to run SBS on the same server as AC. SBS has Windows Update Server, Exchange, SharePoint, etc. I think your just going to slow things down even more. I typically tell the Doctors I work for to just get Server 2008 R2. It's worked out really well for them (although I wish SQL Standard came with it).
For SQL Express which is capped at 1GB of RAM, it's not going to make a difference on an SBS Standard Server assuming you have enough RAM. SBS is just a much better value for the money. Exchange Server is included. Preconfigured RWA, SharePoint. The CALs are cheaper. If you get the essentials version, you don't even have to pay for CALs. Like Bert, I'd take SBS 2011 over Server 2008R2.
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