You might want to use some monitoring sofware, such as Zenoss, Zabbix, Nagios or Spiceworks, among others.
This type of systems check many aspects (ram, disk, cpu and network usage, status of services, etc) of your machines and can be configured to send different types of alerts when something is outside certain predefined thresholds.
Some suggestions and a summarized comparison between some of these tools were posted here:
<a href="http://itknowledgeexchange.techtarget.com/itanswers/free-infrastructurenetwork-monitoring-software-for-smb/">Free infrastructure/network monitoring software for SMB</a>
I use several products, but am pretty impressed with 2 I am now testing, Spotlight and Lansweeper