Is the server hosted infrastructure (Amazon EC2/Rackspace/etc.), or is this a server accessible across a WAN link or VPN tunnel? The term "external" can mean different things to different people. If it's a internet infrastructure as a service solution (Amazon EC2/Rackspace/etc.) I'd recommend monitoring the host via SNMP and leveraging User Experience Monitors wherever possible, as WMI is not an internet friendly protocol.
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