CPU and Running processes
I use the classic: top. Installed as standard in our distributions, It will display the processes running and sort them according to certain criteria.
Here for example that the command returns on my server :
By default, the command will sort the programs by their CPU usage. To change the sorting press: ‘ f’ to select fields to display, then you move with the arrows. The SPACEBAR to display a field then ‘ s’ to use the field as default sort.
Memory usage
To get some extra RAM reduce the graphics memory to 1MB using raspi-config tool:
Another great tool:
Disk Usage
Webmin
There is always webmin that will give you some additional info: