redundant bandwidth/isp to network?

I’ve got 3 Xserves, serving afp, vpn, dns, webdav, filemaker server and 1 Quicksilver running * VoIP server (YDL 3.0). Managed gig switches. T1 line to pub internet, 30 local users, 3 remote sites (20 users 2 dif states w/ T1 lines to public internet) & 6 soho users in 6 other geographic locations w/ mix of Cable and DSL service.

I’m thinking of getting a cable line installed to use as a backup bandwidth provider, and possibly to load share? some of the current traffic. Will this work? Should I just put down the pipe and save $150/mo on the cable line?

If it’s possible, does any one have any links to further info/setup?

Thanks for your time,
greene