My ISP seems to overcharge my bill saying I am using extra bandwidth? How do I stop this from happening..is it something to do with limewire??

I use DSL with 256k up and down.

I need some serious help or else I wont be able to use Limewire no more.

On my ISP Page they say this:

*Monthly Charge includes the following:

• Unlimited Internet access (3 Gb per month transfer)
• Free software including Web browser
• 5 E-mail boxes (additional mail boxes available)
• 10 mb storage space for personal web hosting
• Usenet News – Access to thousands of news groups
• Free technical support
• Additional $5 per month per 500 mbs of transfer

But how do I know if I go over 500mbs of transfer or 3gb per month transer and what do those things mean?