Hi there.
You will need a cable modem supplied to you from Virgin Media if you are using their Cable services, as I seem to see you are intending to do.
You can only use this modem as it is registered for your address so no other modem will work for your connection at your address.
The diffferences between ADSL and Cable modems / routers is that ADSL equipment has a modem already built in and is intended to convert digital and analog signals via your phone line.
A broadband modem is used by Virgin media (NTL Telewest) as it does not use the phone line but instead the cable network.
If you are buying a router, you will need a normal cable router to connect into the supplied modem. You then connect your PC's to the router.
Some router suggestions can be found here:
http://www.cableforum.co.uk/board/87...info-here.html