Alborg, so it comes down to cost. For a system like AC there should be no additional costs for hardware to run SQL Server Express.

Also be reminded that if you buy Small Business Server you get SQL Server with 5 CALS. If you are worried about concurrency issues you should rest assured that ADO.Net address much of that issue because it is based on the "disconnected data set"

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In a disconnected dataset, the database connection is used only for retrieving or updating data. For other things like browsing through records, adding records, or even changing existing records, no trips are made to the database. That's all. You can even use ADO's Update and CancelAll methods to have a disconnected recordset. Or you can use the DataSet of ADO.NET for the same.

This means that the issue of concurrency is reduced many fold. When you need a "Patient" record it connects to the database gets the record and disconnects.

Please read the following article:

http://aspalliance.com/836_An_Insight_into_ADONET

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