PIX 501 Subnet Change and DMZs

I have a pair of Cisco 501’s. Both on the same subnet (obvisouly not physically) – 192.168.2.x

I now want to create a DMZ with one of the PIX’s so I need to change the outside interface to something like 192.168.3.x and leave the other PIX at 192.168.2.x

The ‘new’ 192.168.3.x will not allow me to change the outside interface due to the disable DHCP server configuration.

What am I doing wrong? Or should I just keep the outside interface DHCP and let a DHCP server give it a 192.168.3.x lease?

Or is there some step by step somewhere on setting up nested firewalls that I need to read up on?