Massive bandwidth usage

Since purchasing a new iMAC, my bandwidth usage has gone through the roof, while I expected a fairly sizable chunk of downloads for various updates the system seems to have generated a mind of its own.

For example, went to sleep after nightshift i think only the itunes window open but idle on the desktop.

I have my suspicions that Mail and RSS were the culprits but have no way to prove this or find out if it is anything else. I have now disabled both of my email accounts (GMAIL) and changed RSS to manual updates (never actually use RSS)

But during my sleep the computer had chewed up over 300MB of data! Even if mail were the culprit given I consider this excessive.



Any help available out there, or anyone with the same experience

Thanks
(New to Mac)

iMac 21"
3.20GHz Intel Core i3
4GB 1333MHz DDR3 - 2x2GB
OSX - 10.6.4