You best bet would be ISP modem to router (wired) and let that router do NAT (IP Sharing in Airport Extreme). Any router after that should NOT being doing NAT (connection Sharing). This way you only have the main router doing NAT and avoiding double NAT in local Sharing.
On my setup (with two routers) has a network setup:
Cable Modem->D-Link wired router. Connected to the wired router is my desktop G5, Linksys VOIP (Vonage) modem and Time Capsule. I setup the Time Capsule up to be on the same subnet by setting it up to "Bridge" mode. This way my Mac Book Pro can see the Time Capsule and both the G5 & Mac Book Pro can share music/video with each other because they are the same subnet. So if I look at the IP (System Preferences->Network) in the G5 & Mac Book Pro they are on the same subnet by having similar IPs given out by the wired D-link router.
So you best bet would be ISP (ISP usually gives just one IP) modem wired to the first router. This router should be doing NAT for the network. When you wire to the Airport Extreme in the Extreme turn off NAT by setting it up into "Bridge" mode. connect a wire between the Airport Extreme WAN port to LAN port on the ISP router (connected to via a wire). This way if you plug into one of the Airport wired ports only the main router will being dishing out the local network. This why if you ever have a device wired to the main router and another to the Airport Extreme they both will be on the same subnet for local sharing and both workstations will have IP that are in the same subnet.
Mac Pro Dual 2.8 Quad (2nd gen), 14G Ram, Two DVD-RW Drives, OS X 10.8.3
2006 Mac Book Pro 2.16 (first Gen) OS X 10.7.4
2TB Time Capsule, 2 TB
32G iPhone 4S Black, iPad (3rd Gen) 32G Black