I only have 1 dedicated server currently, so I use
www.pingdom.com for server monitoring. When I signed up they were having something like a 75% off sale, which worked out quite well for me.
I haven't looked into Pingdom prices for when/if I exceed their basic package, but if at some point it is no longer cost effective to use them for monitoring I will be checking out Nagios if that time comes.
I've never heard of Cacti until now... It also looks like a good product, thanks for posting that link.
