Two MRTG threads within 10 minutes. I'd say it's time to make some sort of sacrifice -- or buy a lottery ticket. Or, perhaps both!
Yes, MRTG is the way to go, snog. It can be tricky for a newbie to install. If you don't have access to the router (some dedicated server providers won't let you), you can always sniff the Ethernet port which is almost as accurate. (Although, if you have several computers involved, you may inadvertantly measure LAN traffic. If it's just one box -- that's the way to go.)
I will save you some time on this one. It took me a lot of Google-ing to find how to sniff the Ethernet port. When you get to the part where you make the config file, you will want something like this for your target (or "interface"):
You will, of course, change "server" to whatever you are naming the group. It can be anything, as long as they are all the same. Regardless, that will sniff the Ethernet port for incoming and outgoing.