WDS vs. "Extend Network" option

Hello mac-help,
I have an Airport Extreme Base Station (Fast ethernet, not gigabit) and two Airport Express Base Stations (G). The purpose of one is to use AirTunes (it is plugged into our speaker system). The purpose of the second is to extend the network to the upstairs. I have tried a few ways of operating this network. First I tried just having the AEBS broadcast, but I checked the box that said "Allow this network to be extended". I used the Expresses as they were designed to do. One day, however, I noticed that my signal strength directly next to one of the Expresses was a lowly 57%. So, I explored the option of a WDS network. Now the AEBS is my main, and the other two are relays, one of which does the AirTunes connection. This network, however, is highly unstable. The AEBS often goes from a green light to flashing amber and back to green for no apparent reason. I just read on The Unofficial Apple Weblog that getting two N express stations to replace my current G ones may be the answer. So, I just want to make sure that I am not completely crazy. Which should I be doing, extending my network, or WDS, or shelling out more cash to Apple?

Thanks.