You have most of it set up OK, but what do you mean by roaming network? Is this a second WIFI network or access point? What are you using the airport express for?
The new AP Extreme WAN port is cabled to the modem.
The new AP Extreme is set up as router (as it comes from the factory) and you set up WIFI on it (have all your wifi devices set to this network).
The new AP Extreme is dual and multi band, slower wireless devices are not going to impact the high speed wireless. All your devices can connect to it wirelessly without issue, you really don't need anything else.
If you are using the time machine for backups, connect it via ethernet cable to the new AP Extreme. Turn its wireless off if you haven't already.
You can connect any of your devices via cable to the new AP Extreme, do not connect to the MODEM. The only thing that should be connected to the MODEM is the WAN port of the new AP Extreme. For best results, connect your Xbox via ethernet cable to the new AP Extreme.
If you want to set up a second WIFI network (what I think you mean by the roaming network), set your third gen AP Extreme to bridge mode, give its SID a different name than the new AP Extreme SID. Its WAN port should be connected to the new AP Extreme via cable. You can then set your slower devices to use the thrird gen AP network instead of the new AP Extreme network, as load sharing. You may see some improved performance if all your devices are streaming video at the same time.
You may be running out of ports on your new AP Extreme. You can use spare TC and the older APE ports, but it may be better to pick up an inexpensive 8-10 port GB ethernet switch. You would connect the upstream port of the switch to one of the new AP Extreme ports, and connect the devices that there is no room for on the AP Extreme to the switch.
Here is an example of a decent switch:
http://www.newegg.com/Product/Produ...&cm_mmc=TEMC-RMA-Approvel-_-Content-_-text-_-