Re: X Box Live set up via secondary modem ?
*If* Virgin supply you with a second modem, it would work that way. However, as a matter of policy, they only provide one modem per house, and buying your own is not only against the Terms and Conditions of service but probably wouldn't work and may cause Virgin to take action against you.
What you need is a cable compatible router (such as the excellent Linksys WRT54G series). Connect that to your modem and connect your PC/Xbox/Any other devices to that.