Backup Internet Link

Hi all
I am a bit confused on how I might be able to achieve a backup
Internet link in my current environment
A bit of background
LAN with multiple subnets, the default route goes to the Cisco PIX
515
firewall on 192.168.100.37
The Cisco PIX 515 has an Inside i/f, a DMZ i/f (4 in total, 1 in use)
and an Outside i/f
On the PIX all HTTP and HTTPS (basicaly any internet) traffic is set
to go to the proxy server in the DMZ (192.168.1.10) via access rules.
This is a Linux server running Squid
Access rules set on the DMZ i/f are then set to allow traffic to the
outside interface for Internet access
The current outside interface is part of a /29 subnet assigned to us
from the ISP. x.x.x.41 is the routers ethernet i/f and x.x.x.42 is
the
PIX, the other IP's are all assigned for various things like mail,
webmai, vpn etc

I was wondering how I can implement a redundant internet link. The
new
link is in and connected to a Cisco 837 but I don't have this
interconnected to anything in the network at this point in time. This
link will also be used to access a remote Data Centre site used for
disaster recovery.


I wanted to know
1. How I can make this part of my network so it is secure
2. How can I make the network switch to this connection if the
primary
link fails


Any suggestions would be most welcome


Scooty