I run Cacti on my Pi2 at home (I run a Cisco router, switch, and wireless access point). I use it to tell me things like the ADSL SNR, line attenuation, calculated maximum download/upload speeds, actual bandwidth usage, memory and processor usage. It also tells me how many clients are connected, and to which radio, on my Cisco AP.
Previously I've had Cacti also monitor a laser printer, having it tell me how much toner is left, how much paper is left, how many pages have been printed from it, the number of jobs in the queue, etc. I only don't monitor that now because said printer died.
Cacti uses RRDtool, just like MRTG does. Try using Cacti, and ensure you're polling using the more efficient Spine poller. If you have a Pi2 already, it can't hurt to try setting it up with Cacti and see how many devices it can poll without being able to complete a polling cycle (5 minutes by default, but if you don't need data with 5 minute resolution, you can tell it to poll less frequently).
Cacti can be installed using "apt-get install cacti", and that should cover all the dependencies too, but might install an older version. You can download Cacti from their website - http://www.cacti.net
I'd definitely recommend using a USB hard drive for storing Cacti's data. Save your SD card a lot of writes, and give yourself plenty of capacity.
It's also worth noting that yes, I only have a few devices for Cacti to monitor at home, but that Pi is also a Samba active directory domain controller, a DHCP server, a USB print server, an FTP and TFTP server, a CrashPlan automated backup server, and it also runs Serviio, which is a DLNA streaming media server, and I'm able to stream full-HD Blu-Ray rips from that Pi to my PC or PS3 without stuttering, even when Cacti runs its polls, or when CrashPlan is backing up or compressing old backup archives. The Pi2 has a decent amount of grunt. Give it a shot.