SQL 2005 not using all the memory that is allocated to it

Ok.. Kindly confirm the Min and Max values specified in your SQL Server instance.
Also let me know whether you have one or more instance in your server.

What version of SQL Server  2005 are you using?

http://www.microsoft.com/Sqlserver/2005/en/us/compare-features.aspx

SQL Server will use some memory as per the memory component/requirement of its architecture. With user defined tables when one does SELECT/INSERT/UPDATE/DELETE all occur in RAM. So, when lots of these happen especially SELECT fetching lot of records from big tables, your RAM usage will increase.

Also refer:

SQL Server memory architecture
http://blogs.msdn.com/teekamg/archive/2007/11/05/sql-server-memory-architecture.aspx

Memory Management in SQL Server 2005
http://www.gre-sqlserver-solutions.com/Memory.html
http://searchsqlserver.techtarget.com/searchSQLServer/downloads/Guru_Guide_to_SQL_Server_Architecture_Chap%2011_AWL.pdf
Dynamic Memory Management
http://msdn.microsoft.com/en-us/library/ms178145.aspx

How to adjust memory usage by using configuration options in SQL Server
http://support.microsoft.com/kb/321363
SQL Server memory usage
http://blog.colinmackay.net/archive/2008/07/20/2996.aspx
SQL Server Memory Usage Grows and Not Shrinks
http://www.sql-server-performance.com/faq/sql_server_memory_usage_p1.aspx

rrjegan17:
Ok.. Kindly confirm the Min and Max values specified in your SQL Server instance.
Also let me know whether you have one or more instance in your server.

Min 9500
Max: 10000

Only 1 instance running

ong-hh:
What version of SQL Server  2005 are you using?

SQL 2005 Developer Edition with SP2