I am trying to set up a small server for a computer lab at a conference I'm going to be involved with in a few months. The system will need to provide firewall/proxy/NAT, print/file serving, and similar jobs. I have an old Celeron 533 I'd like to dedicate to the task, but I do have a problem with it: it only has a single PCI slot and no onboard ethernet. Can NAT be reliably and securely done with any sort of shared network? I would think not: that DHCP requests and broadcasts would go to both my server and the outside network. I suppose my next option is a USB ethernet adapter, if that would work well.

I would go the USB NIC route.