I'm kinda new to this so here goes.
Is there a best pratices for wehter you should put your router inside or outside your F/W.
Easily, actually. You can configure your firewall as a layer 2 device, so it doesn't even touch the IP information in the packet.
All, in all, honestly, as someone said above, it depends. The perfect solution is router -> firewall -> (DMZ)/router -> firewall -> corporate LAN
Your outside router can do basic natting for your DMZ servers and stuff. as well as some rough access control with access lists. The firewall behind the router can act either as a layer 2 or 3 device (I think), then your internal router actually does your PAT'ing for the corporate LAN and the firewall behind that has some really buttoned up access-lists (at least in my understanding).