I use pfSense as a gateway to my home network and have no real idea how powerful a system I need... I guess I need to monitor various values to see how well pfSense is coping and if necessary upgrade.
What do I need to look at?
The dashboard shows your CPU and memory usage. Watch your CPU usage when you run a bandwidth test.
You can also go to the shell and run 'top' to see how much CPU is being used.