"The onboard NIC is not in use. The PCI card is assigned to 192.168.0.45. The device extender I need to hook up has an assigned address of .17. Would it be at all possible to make a crossover cable, plug the device extender into that unused NIC, and have the whole mess actually function. My main worry is configuration of that NIC and whether or not the rest of the network will be able to see that .17 address."
You would need to use ICS and bridge the two nics, as suggested by rushonbye. The onboard nic would need an ip address assigned as well. As long as the ip addresses are static and there are no conflicts, there should be no problem with use of the .17 address. Theoretically, it may work. However, your computer will act as a "gateway" to the extender so alot of it depends on the software that needs to access the "extender". The extra hop to the extender would be my only concern, based upon using proprietary software. If the software expects the extender to be on the same LAN (not needing to go through a gateway), then it may not find the extender. It can't hurt to try, so long as any data being sent to the extender will not be lost if the connection cannot be found.
This link may help with ICS setup: http://support.microsoft.com/kb/309642