What distance to expect

I have just installed a wireless Belkin 11g adsl router and a wireless
Dynamode PC card. (Just for information I did not need a cable to
configure the router as some suggest - it works fine across wireless
as it hunts around).
I would like to get a laptop working down the garden. I've seen the
Belkin USB for about £20 (here in the UK) but not sure whether the
distance would be any good.
I actually purchased a PCMCIA card in error that might have done the
trick but the new laptop has no port so I'll have to go USB. I don't
want to spend a load of money but would the USB devices with external
looking aerials be any better than the Belkin £20 job ? The laptop
will be roving so ideally the smaller the device the better. Best
guess is that the laptop will be in a wooden summerhouse down the
garden with a 100 foot of air to the house and a single brick wall
before a product over this sort of ditance without using a special
aerial (i.e. out of the box solution) would be appreciated.

Thanks