RackSpace offers 'managed' accounts, and as such they offer proactive monitoring of your cloud servers. As a RightScale customer, I am curious how the Rackspace and RightScale monitoring implementations differ.
RackSpace monitoring is complementary to the RightScale monitoring. It is important to note that they are not the same, and that RackSpace's monitoring is unsupported from RightScale.
If you do need a specific monitoring metric, it is recommended to write a plugin for Collectd or WMI for Linux or Windows images respectively, which is the supported RightScale methodology for monitoring.