Its rather annoying that the thresholds for all the default gauges are global, global is fine but there needs to be a override for individual nodes such as in the case of SQL servers or Exchange servers that run high memory usage on purpose.
Currently the only way around this is to create a poller and get the value with SNMP. After that you have the old gauge which is sitting there doing nothing for you, and if you want to clear an alarm threshold for that node you have to bump up the thresholds on your entire environment.