I suggest you monitor those numbers daily, and see if they match your own activity.
For example if there's a day you did nothing on the net, but you see a 1-2 gig increase, then something is amiss. Then I would go back to the potential issues listed above (malware, spyware, neighbour, kid on bit torrent, etc).
Right away though, I hope you have followed the suggestions on the wireless security (wpa2 password). Those you should do, regardless of the outcome of your monitoring. It is not a good idea to buy a wireless router and simply plug it in without changing the wireless to some form of secured access.
Also, I think your new Extreme High Speed 10 is the same price as Ultimate High Speed 30. The HS10 has a 100gig monthly limit, the UHS30 has a 70gig limit.
If you don't plan to exceed 70 gigs, the extra speed would be nice. I'm on the UHS50 (100gig monthly limit) and used to have the HS10. The speed difference is noticeable and it's a welcome improvement.