you only need to let one router handle the proper "routing" side of things and that should be the high end linksys router. It can do all your dhcp, you can setup some reserved static ips for your other routers and hardware (dhcp reservation list button), port forwardings and your ddns stuff. It is perfect for it. On the other hand you want to tax the shub as little as possible then you don't have to worry about anything going wrong. Having weak links in your network isn't good.
It should litterally be a check box job and you can untick dhcp on any routers (inc shub) you are going to distribute around the house. This means that all your internal ip addresses will be handed out by your core router (the 6500). I would keep wireless switched on on the 4200 and put it at the opposite end of the house. Set the same ssid on both routers and use different channels and that way your have got 2 kick ass routers giving your full strong wireless coverage. You can use the shub whereever you want as a switch with dhcp, wireless, and FIREWALL disabled.
Also, one of the many joys of having a Linksys router is that you can set your dns servers. I have been using google dns servers (8.8.8.8 and 8.8.4.4) for years and they are great. When there are issues with VMs dns servers and you get other users on the forum moaning they can't get to certain sites (all the shub users
) you can sit back with a big grin on your face.