2-3 routers with same SSID. That's how my college campus works. Every router in the zone have the same SSID so the phone just moves to the most powerful router without losing any connectivity.

A single router may not be enough, since you didn't add in your calculation the inside walls of the house, which weakens the signal strength of a router.

I have a b/g D-Link router, and right now, I am at about... hmm 4-5 meters maximum from it, and on my laptop, i got just 3/5 signal bars. Moving outside the apartament (meaning +1 walls) i get just one bar of signal, so the most I can do out of my router is stand in front of the building block, inside the car, and use my router. I said f**k that, and I got a 3G data plan, and skydrive/dropbox as media storage.