My wife's laptop is actually sitting next to the wireless router.
I'm in the living room one floor away from them.
The wireless router is connected by cable to the cable modem and by Ethernet cable to my wife's computer (I print via her machine).
My wife can disconnect her 10/100 Ethernet lead from her laptop and join me downstairs without a problem.
This does mean however that we lose the ability to print remotely although it is possible to set up the router using it's USB connection to wirelessly print from either machine.
So, I think this shows that provided the modem is connected to the wireless router, then you should be able to position your computer (laptop/desktop or otherwise) more or less anywhere.
I hope this helps, there is an article in the latest edition of PCA about wireless, don't know if it helps.
I've been wireless now (the so-called SLOW version) for nearly 2 years and would never go back. It's great.
I hope that helps, if only with encouragement.