How about RAM usage?
As far as the client is concerned, since monitoring is plug-in based, it really depends on what you want to monitor. If you just want to monitor remote services (PING, SSH, HTTP, etc.) the effect will be the same as opening a session to that service. If you want to monitor local resources you can run NRPE (the Solaris binary is about 216K), which can run as a daemon or in inetd/xinet. Additional resources will be needed for the plug-ins that perform the actual checks-- how many checks you perform, and how often is up to you.