Using PERFMON to get memory counters, what you need to check?

Refer to AWE if you have enabled the AWE in thsi case. Another piece of advice is if this is not a dedicated SQL Server then make sure you have monitoring MemToLeave information too, as it manage the transaction log requirements.

Refer to pages/sec counter too as the high rate for this counter could indicate excessive paging. Monitor the Memory: Page Faults/sec counter to make sure that the disk activity is not caused by paging. To determine whether SQL Server rather than another process is causing excessive paging, monitor the Process: Page Faults/sec counter for the SQL Server process instance. If the target is less then the total that means other process/application is contending for memory hence SQL is dynamically releasing the memory to other process.

Finally as you are using SQL 2005 you could take help of DMV's sys.dm_os_memory_clerks to see the content. http://www.microsoft.com/technet/prodtechnol/sql/2005/tsprfprb.mspx link is a good one too.