Q: Extending wireless via Access Point

What is your ultimate networking goal? To create an extended wireless network that covers a roughly square area or one that needs to cover a pronounced rectangular area along the longest side? ... or some combination of both and over multiple floors? Can the interface between routers support a wired connection or must they all be wireless?

Unless they live within 150 of you with no obstructions between the signal quality is going to be very suspect. Unfortunately, the newer 802.11n or 802.11ac AirPort no longer support the older WDS version for wireless network extending. In the older design you can basically daisy chain wireless routers to achieve the greatest linear coverage. The newer models extend a wireless network in more of a wheel fashion with the base station to be extended at the hub and each of the extending base stations along one of the spokes.

Typically you would use directional antennea to beam a signal across buildings. The easiest way to test, whether or not, your friend can get a useable signal is to make Signal-to-Noise Ratio (SNR) measurements of your Extreme at various locations where you either want Internet access or where you will want to place an extending base station. You will want the SNR value to be 25+ dB at those locations to get a useable signal. Remember a repeating base station can only repeat the signal at the bandwidth that it receives it.

If you have a Mac laptop handy, I can provide you with instructions on how to take SNR measurements.

I am using the older style extremes w/express plug in version.

By "older style" are you referring to the 802.11g (dome-shaped) or 802.11n (squarish) Extreme?

If I put my extreme in the attic and close to an air vent, this will only be obstructed by a screen under an awning and across the street to a room with a window closes facing of my place.  It should be 150 feet.

As you can imagine, just having open air between the router and the client would provide the best signal transmission. Any objects between them will cause RF noise and "weaken" the signal. Also the signal drops due to a phenomena called propagation loss. The farther away from the wireless router the greater this loss.

But I am also curious about using a directional antennea.

One such vendor for these is Quickertek. However, these can really add up in cost to a point where it may be less costly in the long run for your neighbor to get their own Internet service.

Sorry, but no. These would require connecting to an already existing external antenna of a router. The AirPorts have internal antennae and would require more invasive measures to connect such an antenna. This is true for either the Extreme or the Express base stations.

Again, sorry but no. Check out the Quickertek link I provided earlier to get an idea of what it would take.

Configure all three base station for a roaming network. Use one of the Extreme base stations as the "main" and connected directly to the Internet modem.

Place the second Extreme in the attic area you described closest to your neighbor's house. Place the Express in your house where you need additional wireless coverage.

...I am picking up older extremes off craigslist and going to try to connect another friend who lives behind me.

With your current configuration you may not need to add any additional base stations to the mix. However, I have personnaly set up a roaming network with up to eight base stations. Apple has not stated what the maximum number would be. The issue will become Wi-Fi interference between base stations that are in close proximity of each other as there are limited number of Wi-Fi radio channels that they can be configured for.