I understand that this is somewhat of a "halfway" answer because it suggests a completely different solution than what you're using, but you did say in the question that you have a 'basic' MRTG config... this implies it wouldn't be that hard to move to something else. My suggestion uses the 'successor' to MRTG (rrdtool) for its graphing, and was made by the same person that made MRTG (Tobi Oetiker if you're curious)
Have you looked at Cacti? It takes some getting used to, but I've done a lot of graphing and monitoring that impressed everyone from developers to network engineers to management, including monitoring an array of Cisco routers, switches and firewalls. I personally know there's a Cacti template already built for a Cisco 2600 that will monitoring just about everything you could possibly be looking for... bandwidth, temperature (if your chassis supports it), CPU, memory, number of IPSec tunnels that are up (if you have any), number of NAT translations built, the list goes on and on.
Here are some examples of Cacti templates that are already built... some of these posts answer your question as well, because they discuss the SNMP OIDs needed:
http://forums.cacti.net/about5603.html (Cisco memory and CPU)