Check out Cacti and RRDtool. You can use them to poll, record, and display aggregated results as graphs the performance of just about anything on a server. There are plenty of templates available to get you started recording server data.
From RRDtool's webpage:
RRDtool is the OpenSource industry
standard, high performance data
logging and graphing system for time
series data.
From Cacti's webpage:
Cacti is a complete network graphing
solution designed to harness the power
of RRDTool's data storage and graphing
functionality. Cacti provides a fast
poller, advanced graph templating,
multiple data acquisition methods, and
user management features out of the
box. All of this is wrapped in an
intuitive, easy to use interface that
makes sense for LAN-sized
installations up to complex networks
with hundreds of devices.