Using Perfmon on Windows Server 2008 R2 Which counter value should be used for physical memory usage

First you need to understand that RAM usage is related to but not the same as memory usage. See my answer to this question for an explanation of how memory works in windows. That being said the counter that measures physical ram in use (commmit charge peak) is \Memory\Committed Bytes. You should see page faults long before the system actually uses up all the physical ram so check for page faults as well.

Before you start working on ram issues you should watch this webcast on how to troubleshoot memory problems in windows. Also see Pushing the Limits of Windows: Physical Memory

Is this a 64bit server - do you have the lock pages in memory local policy enabled? SQL is likely consuming the rest of your memory If you look at the perfmon counters you will see the memory allocation

Here is an article that explains it in depth

You can also view the counters in SQL

SELECT
    object_name
   ,Counter_name
   ,cntr_value
   ,ROUND(( cntr_value * 8192.0 ) / 1048576, 0) AS cntr_value_MB
FROM
    sys.dm_os_performance_counters
WHERE
    object_Name LIKE '%Buffer Manager%'
    AND RTRIM(counter_name) IN ( 'Free pages', 'Total pages',
                                 'Database pages' ) 
UNION SELECT
    object_name
   ,Counter_name
   ,cntr_value
   ,ROUND(( cntr_value / 1024 ), 0) AS cntr_value_MB
FROM
    sys.dm_os_performance_counters
WHERE
    counter_name IN ( 'Target Server Memory (KB)',
                      'Total Server Memory (KB)' )