Internet usage meter?

I am planning to change my Internet tariff. At the moment I have limitless 256 speed. I would like to change it to 556 but only with 3 giga bytes per month.
I do not download much but my wife listens radio all day and we speak to children in UK via Skype. Hardly turn the PC off. I have no idea if it would be a beneficial change.
Your advice and a software you can advice to measure the existing usage is more then appreciated.
Thank you.

Here's a good little freebie that will monitor your bandwidth useage..

http://codebox.no-ip.net/controller?page=bitmeter2

it monitors in hours, days and months. (It requires .NETframework to work.)

Radio can use a fair bit of bandwidth depending on the bitrate. FM type quality would be a few gigs/month if left on all the time.

Fink,
Thank you very much for the info and the link.