How to measure network equipment usage effectiveness?

["Lee, Steven (NSG Malaysia)" <kin-wei.lee () hp com>]

Hi all, may I know how you guys measure the network equipment usage effectiveness? In what situation you will buy new 
network equipment instead of using the existing equipment? Any clue to share? Should we only upgrade/replace the 
equipment once the max PPS is reached? Is there any tools other there can measure this?

Regards,
Steven Lee