The biztalk user does not necessarily need to be an admin on the SQL Server.
There are a number of domain accounts that you need to create that provide secure access to the Biztalk databases. The security on SQL server is setup for you when you install Biztalk.
The user account you are logged on with to install Biztalk must have sa privilege on the SQL server in order to create the database etc.
You need to follow the instructions in the Multi-Server installation guide found here:
[url]http://go.microsoft.com/fwlink/?LinkId=46922[/url]