I plan to purchase an Airport Extreme to extend my wireless network into my yard
Is your main wireless router an Apple "n" device? This is what you will need for the AirPort Extreme to be able to "extend" the signal.
While the Extreme will provide a bit better range in the direction that it is pointing when it is mounted vertically, you still need the Extreme to be able to pick up the signal from your main Apple router to be able to extend it effectively. For extending, it will work best in its normal flat orientation on a desk or shelf.
Apple claims up to 25% better antenna range with the AirPort Extreme over the AirPort Express. But, if an AirPort Express won't provide enough coverage now, it's hard to know whether the slight improvement of the AirPort Extreme will really do that much better. At best, the signal will still be quite weak in the yard. Whether the signal will be usable is another question that won't be answered unless you try, so you might want to understand the store's return policy in case things don't work out as hoped.
It is possible that you may need to utilize an ethernet connection from the main router to the specific area where more coverage is needed and place the extending device at that location. If you can't run the ethernet cable, you might want to take a look at a pair of ethernet powerline adapters to accomplish the same task by sending the signal over the AC power lines in your home.
If you already own an AirPort Express, you could use it for this type of purpose and be assured that it would do a better job than trying to extend using wireless only with an AirPort Extreme.
