[CentOS-virt] xen host system capacity utilization

Hi, I'm wondering what the proper way is to determine the utilization of host resources when running a couple of VMs. I know xentop but the man-page isn't exactly helpful in explaining what the values it displays actually mean. What I'm looking for is a way to say "Ok, i have 4 VM's running and right now they utilize 40% of the cpu resources the host has, 50% of disk i/o and 30% of network i/o" so that I can determine if and how many additional VMs I can run on this machine? What is the proper way to accomplish this?

Regards, Dennis