WiFi Internet problems

Hi all,

I've recently upgraded my internet to 20megs from Virgin Media here in the UK. My setup is pretty straight forward:

Cable Modem --> Airport Extreme --> Shares Internet

When I do a Speedtest from speedtest.net on my Macbook I get 18-19megs download, everywhere in the house. I also have an iMac upstairs, and a Windows box downstairs and both of these score less than 6megs a second. I've tried the Macbook directly next to both these desktops and I am seeing the same result every single time. Macbook = 20megs, iMac/PC = 6megs.

Both Macs are running 10.5.5. The PC is running XP.

Any ideas? It makes no logical sense to me!