Unfortunately, the internet is not a pay per use (like a pay as you go phone, etc).. and you are paying for anything UP TO that block amount.. whatever it may be.
I do agree. it should be much higher.. if not unlimited, period..
Anyways, onto the help for the router.
Thats the model i have, so i can check to see on mine.
Just as an extra percaution.. make sure to change your SSID name.. this will requir re-connecting on all devices though.
Wireless mode.. you should set to whatever you need. If you only have N devices, there is no need to enable G for it, etc.
You will want to be on WPA2-PSK, not just WPA..
Make sure you CHANGE your password from what it was.. the more difficult the better
Personally, i would leave the cyper type on AES only, unless you need to have it on TKIP for some devices to connect. AES is overall slightly stronger.. and my understanding is that TKIP will actually slow down the max speed (though this would only effect internal traffic speeds).
To get REALLY picky.. you start getting into the MAC FILTERING. PRetty much, you put the physical MAC address of the wireless device, into the list.. and it will only then allow THOSE devices to authenticate.. nothing else can.