Okay, from what I got from that, this is what I think you should do. Off the router connect a 8 port switch to it. Then run a line to the new PC. From the new PC run a line to the old PC. As for the file shareing I suggest turning the old PC into a FTP server, and just don't give out the IP, or let anyone have read write access. Also if you share the printer your good to print from any comp.
Thats the way I would do it.
I don't see the point, really- I should have mentioned that this existing "network" isn't really a network at all. The router only allows sharing of the broadband connection; nobody eles here has access to my PC or my printer over the network(damn good thing, too- I'm the default "IT guy" around here, that's how noobish and non-technical my loft-mates are

Wouldn't it be simpler and cheaper to just spend the extra 60 bucks on the 2-port mobo and connect the two machines via crossover cable? I'm just trying to create my own 2-
PC network that's connected "downstream" of the house router.