Total RAM % from perfmon Windows Server 2008 R2
Solution 1:
Sadly, there is no default Performance Monitor counter that does this. You will need to consider using WMI to tally the total RAM in a server, then totaling the amount of used RAM versus free RAM, storing that data as it is collected, and then presenting it in a pretty way.
However, that kind of thing has already been done for you in the form of the basic features of most NMSs.
P.S. If you're extremely adventurous, you could try writing custom perfmon counters.