I guess the question is, when you say you want the network to use the "real" IP (registered addresses) do you just mean you want them to be able to surf the web or have web clients access resources on your network using these addresses, or for some reason you really want to use the registered addresses on your internal hosts,(God knows why).
If the former, then simply leave your private IP addresses on the inside hosts, and configure NAT and PAT on the outside interface of the Firewall. You can use one address or a pool of them for your clients to surf the web, by specifying this range on the outside interface of your Firewall. For external clients to access internal resources, One to One NAT translations to internal host addresses, and inbound permiting Access Control Lists will suffice.
If the latter (don't let them make you do it:) then you would simply use an extra address from you assigned Class C, on the inside Interface, thereby turning you nice new Firewall into an overpriced router.
Hope this helps
Chris Weber CCDP