I've found god to be easier to setup and configure than monit.

http://god.rubyforge.org

this looks just about right. How does it compare to something like cacti? Has anyone used this? How does it feel? What else should I be looking at? How much mem and disk space does it end up taking up with default config? Recommended tweaks?

Munin has worked great for me. I've written a few node scripts for monitoring Passenger (aka mod_rails) and Beanstalkd and it was super easy.

Ex: http://gist.github.com/37525