Yes, the problem we have here is that the Arris is a modem/router combo, so turning the wireless portion of the unit off isn't going to solve the problem. With this setup, turning off the wireless on that unit does just that, but it doesn't turn off the DHCP and NAT within the unit, what we'd need to know here is if there is a way you can turn off those functions in the arris, this way it'll work like a dummy modem and you can have the buffalo handle all of the DHCP/NAT/Wireless connections. I would look around in the arris config and see if there is a way to do such. If there is no way to turn those services off, then even if you hook up the buffalo you'll be double NAT'd which isn't good.
Or another option would be to turn off the wireless part of the arris, let it continue to do DHCP and NAT, and configure the buffalo to be a wireless access point only, so this way it wouldn't handle anything but the wireless connections of your network.