Late to this thread, but here's my two cents.
If you want a good WiFi coverage, the best way is to get more Base Stations. I personally do not recommend getting WiFi repeaters, in fact, I recommend you upgrade your current AirPort Extreme (saucer) to one of those new AirPort Extreme Base Station (802.11n) or Time Capsules (802.11n), or at very least AirPort Express Base Station (802.11n).
Next, you might need multiple Base Stations. All of those needs to have the same wireless network ID, so when you travel from one base station to next, you can remain connected. To achieve this, there are two ways. The first one is connected via ethernet cable known as Roaming Network. The second one is connecting each base stations wirelessly, known as WDS.
Also, I strongly against using ethernet over power outlet (or ethernet over power line), the main reason being is doing so, you are intentionally ruining the sinusoid wave form of your electricity.
Well, that was my two cents, not a cheap option I know.
(As for me personally, I have 3 AirPort Extreme Base Stations, and 1 Time Capsule Bast Station, all 802.11n, and Roaming across the house.)