Firewall between router and switch?

Hi, after doing a little research on firewalls Ipcop sounded ideal, however I'm just trying to see how everything will work before I jump in and try to do it.

I currently have a cable modem connected to my router, routers uplink port connects to my switch, and 3 pcs connect to switch, I was hoping to put ipcop between the router and switch, do you guys reckon thats the best place to put it?.

The problem I'm having is trying to work out how it will all work, I mean all PC's use XP as the OS and printer and file sharing is enabled.
Well currently my router provides NAT, hence all PC's can use internet at once with the one public IP that I have, i'm presuming I would have to switch NAT off on my router and then enable NAT on ipcop?, the more i think about it the more i get confused!.
I was thinking that it wouldnt affect my printer and file sharing as the switch will handle all local traffic, its only the traffic heading for web and incoming web traffic that will go thru firewall, right?.

This is my planned new setup:
I've got a new PC ready to be installed with Ipcop, has 2 nic's, specs are ok (700mhz celeron with 512meg, 10GB HD).
Well routers uplink port will connect to one nic, then one cable from other nic to switch (netgear switch automatically detects connections from uplink or something), and then i'm hoping after a lil tweaking everything will work.

Sorry for the long post, I hope it all makes some sense, if not just ask to clarify parts and I will.

Any help much appreciated, hoping to get this sorted today!.