That is true. Depending on the edition, you can usually allocate 4GB or more:
http://msdn.microsoft.com/en-us/library/ms143685.aspx The workgroup edition that comes with SBS 2011 Premium Add-on tops out at 4GB.
The SQL Server Standard is 64GB.
The reason I said it might be less efficient would depend on the current hardware of your server. If you have a RAID 1 of 500GB hard drives, that might be too slow for two operating systems.
A way you can maximize the performance and keep AC support is to RAID 1 some SSDs and store the database on there. That'd make it very fast.
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