Realistically? That depends on a lot of factors.
It depends on the router.
It depends on the type of antenna on the router.
It depends on the position of the antenna on the router.
It depends on the materials in the walls of your house.
It depends on if your neighbers also have a router.
It depends on if you have a running microwave or other high frequnecy devices in the house.
It depends on the size of your house.
It depends on the relative position of the router on the second floor.
Generally, though, a good router will reach your iBook/PowerBook all around your house if it's placed somewhere near the middle. I know some people that can walk out on their porches and into the yard and still get usable reception, but I also know others who have trouble just 20 feet away.
I can't speak much for personal experience because my current housemate's wireless hub is a cheap, old piece of crap. Last summer, though, I shared a townhouse and could get access to my PowerBook from in my car, parallel parked across the street.