Wifi Help

My wifi speed has fallen lately, and I'm wondering what I might do?

I've got a 2007 Airport Extreme Base Station (802.11-n (draft)), running in 2.4 GHz n (b/g) mode, with 8 devices connecting: two iPhone 4, iPad 2, MacBook Pro, iMac (ethernet too), Kindle, Tivo, Canon Printer

Speedtest shows about 10 - 15 Mbps download on the iMac, but about 1 - 6 Mbps on the MacBook Pro over wifi. The iPhones suffer similarly. I've seen double or triple the performance in the past. That was before the iPad, Kindle, and printer.

The iMac also shows up to 15 other networks from around the neighborhood.

Are there tools I can use to test my AEBS / wifi setup?

Do I need more routers to support up to 8 devices? (though only about 3 are active at a time)

Would a new AEBS, with its dual antenna support help?

Thanks!

Turning the printer on doesn't fix things.

Speedtest results are highly erratic, which suggests general network interference. Though Airport Utility shows what I interpret as low noise and good signals.

MIght be a mixture of noise and SpeedTest.net variability.

I think it's my wifi. Wireless performance is 5x or worse slower than ethernet.

iStumbler shows only 75% signal on my laptop sitting a foot from the router. And there are five other networks with 10% signal. This weekend I've got to turn on wireless devices to see if it helps. Maybe I'll buy a new AEBS on Black Friday to move to 5GHz n for a couple of devices.

Is there an easy tool to measure LAN performance besides iStumbler?