not sure why you would put the router behind the pc.
basically I've done this kind of thing before. my design was
inet----router --- switch --Linux box(running proxy server,sniffing tools)---switch---pcs
(reason i had a switch in front of the Linux box was to allow an alternate path {link between the 2 switches} so that in the event of the Linux box going down {not likely :D } a quick gw change of the other pc's would fix the issue)
if you don't have a second switch i would just do this (could also have the router in between the inet and Linux box)
inet-----Linux box(with all appropriate routes etc, sniffing tools etc)----switch----pcs
if all your interested in capturing is web traffic then just have Linux box sitting on the same segment as the other pcs but set the other pcs to use it as a proxy server.
If you have the luxury of a switch/router that allows port mirroring then it becomes even easier :D