Measure CPU and Memory usage while running pgbench
I show some tools I know.

pg_activity  is a simple monitoring tool like top.

pg_statsinfo and 
pg_stats_reporter are  high quality utilities that monitor statistics and the activity of PostgreSQL as well as Operating system.

There are some general purpose monitoring tools such as  Cacti and Munin. Those support postgres by providing modules and they can easily be used.

BTW, I know a great book: The Art of Computer Systems Performance Analysis. This gives you some hints for benchmarking.