How Do I Change a Machines IP Address?

[Dave Martini 1]

I need to change the IP address of a RHEL WS 3.0 machine.

I changed 

/etc/sysconfig/network

with the new IP address

and restarted the network service and did a 

ifdown eth0
ifup eth0 

But that did not help.
I also rebooted the machine and ifconfig -a still shows the old
IP address.

I also did a 

ifconfig etho newipaddress
but when I restart the network service it reverts back to the old IP address.

What other files do I need to edit in order to change a machine's
IP address?

P.S. I know I can use netconfig but want to understand what files
it modifies.

Thanks,
Dave Martini
LLNL