airport extreme comcast

Is the "modem" provided by Xfinity just a modem or a combination modem and wireless router? How do you have the AirPort Extreme configured with this device? That is, is it connected directly by Ethernet or is there a wireless connection between them? If wireless, is the Extreme configured to join or extend the Xfinity's wireless network? Finally which exact modem of the AirPort Extreme do you have?

Since you have the 3G Extreme connected to the Xfinity router by Ethernet, you have the basis for a roaming network. This configuration will basically provide you with an extended wireless network in which you can literally roam between routers with a wireless client and still be on the "same" network.

...could I technically move my extreme upstairs and have it wirelessly extend the xfinity signal? Or is the roaming network a better solution?

Only if you can still keep the connection between them using Ethernet. Unfortunately, most non-Apple routers are not compatible for extending via a wireless connection between routers. In your case, the roaming network would still be the best option.

In line with a roaming network, another option would be to employ Powerline adapters, instead of, Ethernet between locations. These adapters take advantage of your home's electrical wiring to provide a pseudo Ethernet. They have come a long way performance-wise but will add considerably to the cost of your network as you will need a minimum of two (one for each router) to create the connection between them.