OK unless you have turned off routing somehow you must have different networks on each interface of a router. Since four of those ports are really just switchports off an internal interface that router should have two networks; 192.168.0.X (.100 for my example) on the "outside" and something else like 192.168.1.X on the inside, let's say .1 for the router.
Router will have to do NAT, use a default route to 192.168.0.1, and should provide DHCP to the "inside" area.
PC7 and PC8 will need an ip on the 192.168.1.X side, SM 255.255.255.0, DG192.168.1.1, DNS will be 192.168.0.1 still. The PC1 will need a static route pointing the 192.168.1.0/24 network to 192.168.1.100 (router)
This should be enough to get you started in the right direction, if anyone notices something I missed please post it as I have to get to work. A dual NAT setup sometimes causes some weird problems, occasionally depending on the limitations of the routers it just doesn't work.
A final question what OS and version are you running on PC1?