If you have more than 4 GB physical memory, then SQL server will not use more than 1.7 GB memory. To make more memory available to SQL server, enable AWE and cofigure Set Max Memory parameter using sp_configure stored procedure. If you do not set max memory parameter then SQL will eventually eat up all memory it can. For instance, if you have 8 GB physical memory, the at least set the max value to 6 GB and leave the rest to the OS.  For more info: Check out this MS Link http://support.Microsoft.com/kb/274750