MRTG is a great packet for monitoring router throughput. For a
sniffer, I recommend Ethereal. Both are free.
If you're not sure if you have a virus, you're obviously not running
current antivirus on all of your machines!
This is what you are looking for:
http://www.eeye.com/html/Products/Iris/index.html
BOM
May not have pretty graphs but might be some mods out with them.
Check out Ethereal, free, uses PCAP library to pull in the packets,
Linux, Windows and probably other binaries available.