You could run MRTG to graph various aspects of the server, such as CPU usage, RAM usage, number of processes. It is very useful for servers without a GUI as it creates images for displaying on web pages.
If you have GNOME installed you can monitor system resources using the System Monitor application.
Alternatively you could run something like this every minute from cron:
top -n 1 -b | head >> logfile
which will log uptime, users, load averages, number of processes, CPU usage, memory/swap usage and the top three resource hungry processes into a file for viewing later. -n 1 runs top once
As you specifically mention you run Ubuntu I believe you can get this monitored by Canonical using Landscape.