Q: Advice on setting up this network

1. Before I get started with this, is there anything I'm missing or forgot about?

You appear to have all the necessary equipment. However, note that what you are proposing is not a form of a roaming network. That is because, a roaming network requires that all wireless routers/wireless access points be connected by Ethernet. Instead you are describing an extended wireless network.

2. Do I setup the Express in Extend a wireless network mode? Will this also allow me to use it to connect the printer?

Yes, the 802.11n AirPort Express Base Station (AXn) would need to be configured for Wireless Mode: Extend a wireless network, and the 802.11n AirPort Extreme Base Station (AEBSn) would need to be configured such that the option: Allow this network to be extended, is enabled. Yes, this configuration would also allow you to share a printer to any client connected to either base station.

3. The Express will be located on the other side of my house, so the signal will already be somewhat weaker there, though there is nowhere conveinet to plug the Express into ethernet. Am I wasting my time by extending the network this way since the signal is already weaker by the time it gets to the Express?

Potentially, due to the signal strength at the location you propose to place the AXn is not sufficient, the AXn cannot "boost" the bandwidth only the signal strength. What you will find is that wireless clients will "see" a strong signal, but have relatively poor data transfer rates. You may want to consider using Powerline adapters if you can't run Ethernet between the AEBSn & AXn. These type of adapters take advantage of your home's electrical circuit to create a pseudo Ethernet one. You can then configure both base stations for a roaming network and not have to be concerned about the signal strength.

1. If I end up making my network an extended wireless network, will it basically still act as a roaming network by allowing me to walk around a house with a laptop with one connected network, except by definition it is not a roaming network because the Express is not connected by ethernet?

Yes.

2.  Can you provide any links to these Powerline adapters you suggested? Ones that will work well with the Airport equipment? Do these really work? Are there any drawbacks to using these?

Here are a few Powerline adapters to consider: 1) Cisco, 2) Belkin, 3) Netgear, & 4) ZyXEL, just to name a few. These adapters have come a long way within the last five years. The primary drawback is that in order for the adapters to create an Ethernet network, they must be on the same electrical subcircuit.

1.) Would I just plug the Powerline adapter into the electrical in my wall and than plug the Airport Express into the Powerline adapter via ethernet CAT6?

You will need a minimum of two of these adapters to create an "Ethernet" circuit between them. You would place one near the AirPort Extreme and connect an Ethernet cable between one of the available LAN ports of the Extreme and the adapter ... and then place a second adapter near the AirPort Express. Finally, you would connect a second Ethernet cable between the second adapter and the Ethernet port of the Express.

2.) Any idea how much the bandwidth gets cut down by going through one of these Powerline adapters? So as an example, if I was getting 30mbps via WiFi (have no idea if this is accurate, I just made it up) could I expect the data coming from the Powerline adapter to be higher or lower than this amount?

I would expect at least 40-50% of what the manufacturer claims that the bandwidth will be as there are a number of factors, but you should still get better bandwidth than you would with trying to extend the network with the Express too far away from the Extreme to get an adequate signal.

I would further recommend that you purchase these through a vendor that offers a money-back guarentee ... just in case they don't work out for you.

1.) Is there some other device that I can buy to get my printer on the network OR do I need to buy another  Express N to do this?

If your printer has a USB interface, then either another AirPort Express N or a device wireless USB bridge should do the trick. I believe either Linksys/Cisco, Belkin, or D-Link carry these.

2.) If I buy another Express N, I will be using the Extreme, and two Express units, so my network will be an extended wireless network and one Express will be set to  Extend a wireless network and the other would be connected to the printer, what will the one connected to the printer be set to?

The one connected to the printer would be configure to join the "extended" wireless network as a wireless client.