1 router, 2 IP addresses, 3 PC's

I presently have a Netgear RP614 router taking a cable connection and sending it to PC#1 and PC#2. Both PC's have thier own IP addresses.
I would like to connect a third PC (Windows ME) to PC#1 (Windows XP) and share files, printer and possibly the internet connection. Is this possible using the same router (two slots open)? Do I need to add another router? Where would it go? Or is there another option altogether?
PC#2 would need to remain its own entity from this connection and keep its individual IP address.

You can just plug the 3rd PC into the router as well.

If the two IP addresses you're dealing with currently are RFC1918 private address space (10.*, 172.16-31.*, 192.168.*) then everything should just work. If either or both of those IP addresses are public addresses provided by the ISP, there may be a little configuration required on the router.