MRTG is probably the easiest to setup. If your router has SNMP (as you mention), to setup it's a single command:
cfgmaker --output=mrtg_myrouter.cfg public@1.2.3.4
MRTG is good for high-bandwidth routers and the likes. It's not great for other data (it can be coerced into graphing most things, but it's a little unintuitive to setup)
For monitoring other stuff I like Munin. I would describe it again, but I posted an answer a while ago here (about graphing disc-usage).
Munin can of course graph network usage, and easily pull data via SNMP (in fact it's the recommended setup for grabbing data from Windows-based servers - run a SNMP daemon on the Windows machine, and have Munin connect to this). The graphs are also prettier than MRG, I would say (clearly the most important factor..)
There's an example installation of MRTG here, and Munin here
