I'm trying to get a better handle on what's happening on my network at work.  I'm currently using Nagios to monitor the big things (Server health, switch health, etc), but I'd like more granular statistics (how much data is coming and going, are any links saturated, etc).  A quick glance shows that MRTG, Cricket, and RRD are good things to start playing with.  I'm staring to play around with MRTG, but I'm hearing rumors of scalability issues and I don't want to get deep into MRTG if I should be putting resources elsewhere.  

A quick sketch of the network is a few internet connections, our router, and a few switches for networking along with a few switches for VoIP as well as our PBX.  I don't want to monitor every port all the time, I instead want to monitor the uplinks of the switches to have a good idea, and look at the switch if something looks awry.

My question to the group is, what do you use / recommend, and do you have anything that sticks out in your head about any of these packages (good or bad)?

Thanks in advance,
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Eric Martin