Use 'ps -ylC httpd --sort:rss' to find process size. Divide number by 1024 to get megabytes. Also try 'top'.
Use 'free -m' for a general overview. The key figure to look at is the buffers/cache used value.
Use 'vmstat 2 5' to display the number of runnable, blocked, and waiting processes; and swap in and swap out.
