How to know how much CPU and RAM I need for new computer?
You could get a 3rd party app like System Pal or iStat. Both apps reside in the menu bar and tell you things like memory used and CPU usage. You can easily monitor either of these apps. I prefer System Pal to monitor memory and CPU usage. It just gives the top processes.

Another option is to use "top" or "htop" from the command line. These processes are like Activity monitor except it runs via the command line.