Hi Bob,
Well, I ain't no expert on this, but I can tell you this:
1. I think that 60% strength at 3 feet means that you're Airport is sending a weaker signal than it should, for whatever reason. 3 bars of strength, at 30 feet, even though it's through a floor is a low signal as well, especially with the 60 % figure at 3 feet. You should be much closer to 100 % strength IMO.
2. Changing channels is relatively easy. Go menu>Utilities>Airport Utility.app, and then you can follow the step by step. Depending on whether you have devices like wireless phones that share the 2.4 Mhz. frequency band, changing the channel can be quite useful. We use channel 4 or 5, and don't generally have much trouble. The Airport cards will read any of the frequencies that the Airport is capable of.
3. Apple recommends WPA Security. Depending on where you live, you may not need to use it. Yes, you could see a 5-30% slow-down with the calculations to encrypt/decrypt the wireless traffic. We live on a dirt road with no neighbors close enough to tap into our network, so we don't use any security. AFAIK, security settings won't affect the signal strength.
From what I'm seeing, I think your Airport may be in the middle of a slow death.
HTH,
Chris