Seeking info on multiple, simultaneous Net connections

[Steve Snyder <swsnyder home com>]

I've just gotten a second Internet connection on a machine that is used as 
a LAN gateway to the Internet.  So now this box (Redhat v7.2 /w 2.4.16 
kernel) has: 

  eth0 - internal LAN
  eth1 - ISP #1 (ISP1)
  eth2 - ISP #2 (ISP2)
  
I've had ISP1 all along and would like to keep it as my primary connection 
to the Internet (it's the faster of the 2 connections).  My first choice 
for ISP2 would be for concurrent use with ISP1, i.e.  to augment ISP1's 
bandwidth.  My second choice would be to use it as a fallback interface 
should ISP1 be offline.  

I currently use iptables to masquerade LAN traffic and to act as a firewall 
on the ISP1 interface.

These preferences leave me with a lot learning to do:

  1. How to integrate a 2nd external connection into my firewall config?
  2. How to use 2 Internet connections (different ISPs) concurrently?
  3. Interface fallover:
    a. Config box such that ISP1 failure is detected and ISP2 takes over.
    b. Config box such the restoration of ISP1 connectivity is 
       re-established and making it again the primary gateway.
  4.  How to direct certain IP addresses to a specific interface (mail
       destined to a specific ISP's mailserver goes to that interface.)
       
I've looked over the HOW-TO on the Linux Documentation web page and don't 
see these topics addressed.  Can someone point me to resources that 
describe the handling of multiple sources/destinations of Internet traffic? 
 

Thanks.