Memory is cheap and often easy to expand in a modern Windows server. But…
While investigating a memory related error on a Windows 2008 based SQL 2008 Enteprise edition multi instance cluster i realized thet using Lock pages in memory prevents the Task Manager from showing the total amount of memory used by SQL Server. In fact what it does is it shows the memory usage for everuthing in the SQL Server process exept the buffer pool.
The total amount of memory used can be found using perfmon and the counter Memory Manager:Total Server Memory More information here, here and here !