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By default, the priority boost setting is 0, which causes SQL Server to run at a normal priority whether you run SQL Server on a uniprocessor computer or on a symmetric multiprocessor (SMP) computer. If you set priority boost to 1, the SQL Server process runs at a high priority. This setting does not make the SQL Server process run at the highest operating system priority.

Based on actual support experience, you do not need to use priority boost for good performance. If you do use priority boost, it can interfere with smooth server functioning under some conditions and you should not use it except under very unusual circumstances. For example, Microsoft Product Support Services might use priority boost when they investigate a performance issue.

IMPORTANT Do not use priority boost for clustered servers that are running SQL Server 7.0, SQL Server 2000, or SQL Server 2005.

P.S.: archive post Dec 8th, 2006