recommended performance alerts

Hi all,
I am trying to configure performance alerts on w2k server boxes. I am pretty much only wanting to deal with server health ie: disk, memory, cpu etc. I have found some resources on the recommended counters to config, but they dont say how often they should be checked (ie: memory paging is recommended, but I am not sure how often it should be configured to check the counter)..can anybody please point me in the right direction.
TIA