What Are Good Performance Indicators

I, like everyone else in the IT world, am now having to justify every little purchase for my equipment. I am looking to have a good discussion on what Perfmon counters can show you about various subsystems (CPU, Memory, Disk, etc.) & which counters tell the best story.
For example, I have a NT4 PDC that is a P1 100Mhz machine with 48MB of RAM. This machine serves up DNS, WINS & DHCP for over 450 users. It needs to be replaced for obvious reasons, but how do I prove it to someone that knows nothing about computers. I can't BS it because the person will verify it with another source.
Everyones $0.02 are welcome.
Thanks,
-MLevy