Redundant internet feeds

The nature of our business requires a disaster recovery solution for all mission critical applications. We have a NT4 network with a proxy server and Raptor firewall. We are presently feeding the firewall with a 128K frame circuit. We are upgrading that, but need a redundant feed. It doesn’t make much sense to use another frame circuit, because at some point they are going to go thru the same building. I would like to add a cable modem access. Can I configure the two feeds in a load sharing scenarion or can I do a fail-over configuration? Another alternative would be cable modem with a fail-over to dial-up isdn. Any ideas?