The amount of system resources our monitoring agent uses on your system depends on the amount of server monitoring the agent has to do. If you have many roles assigned to your server, it will be a bit more busy then with fewer roles.
Here is an example of the running zabbix processes on one of our web servers:
[09-03-11 13:50:09] [root@xyz.com]
[~] # ps aux | grep zabbix
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
We monitor each of the different parameters we keep track of in a different interval. That means for instance that we monitor ethernet traffic every 5 seconds, cpu usage every 10 seconds, but the total amount of memory in your server is only updated once every 24 hours currently. In short, we set the intervals to something that makes practical sense.