One obvious reason is PIX501 has 4-Ethernet port on the Inside, while PIX506 has 1-Ethernet port.
So, you definitely have to modify the config to reflect PIX506E interface numbering.
With PIX501, you typically connect 4 devices directly to the box, or used hub - as long as the total users/clients are within your license scope (32 DHCP address leases with a 10-user license, or 128 with an optional 50-user license, and 256 with an unlimited user license). While with PIX506E, you have to connect the Inside interface to a hub.
When connecting/migrating to PIX506E, you need to assign IP to Inside interface IP. This IP will be used as GW for internal hosts (if connected directly without any L3 devices hosting other segment/subnet).
As for Outside interface, you probably can maintain DHCP setup or static IP.
Other than that, for ACL, you probably need one on your Inside interface, if needed.
More details can be found at:
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/prod_installation_guides_list.html
*Look under Version 6.x
Hope this helps. Pls rate any useful post(s).
AK
