802.11g signal strength informal testing

A buddy of mine in the UK recently purchased a 15" Apple PowerBook G4 (Aluminum). I have this same computer. We're both noticing some pretty abysmal signal strength numbers from these computers, so we're trying to figure out the culprit. Is it the built-in AirPort Extreme cards? Is it the base station? (He has an Apple Airport Extreme BS, I have a Netgear WGR614.) Luckily, I happen to have a PC card 802.11g adapter (in addition to the AirPort Extreme card), based on a Broadcom chip, and made by Buffalo. So, here are the informal numbers from iStumbler, going through each channel available to me.

broadcom pc card:
channel strength
1 59-60
2 73
3 53-55
4 73
5 73
6 74
7 74
8 49-52
9 73
10 53-59
11 50

apple airport extreme card
channel strength
1 54-56
2 68-69
3 68-70
4 52-54
5 71
6 56-58
7 70-71
8 69
9 69
10 70-71
11 58-59


So, it looks like I'll stick with channel 7. Hopefully he'll be able to find a better channel for him. I was originally on channel 11.

It's funny, though. Watching iStumbler, my signal strength on channel 7 is dropping. It's now down to 56. I don't know why...