I don't have a lot of real-time experience with the Aiport Hardware, but I'll assume that it's setup for Consumers so it probably lacks the feature to act as a signal repeater (I don't know if any wireless hubs can do that).
Basically you have 2 options.
Run Cat5 to the 2 furthest points in your house, and set up an Airport Base Station at each point. Set up a Router/Firewall at the Cable source, or maybe have one of your BaseStations do that. Then you can have the long long wires to where you want your BaseStations.
The other more problematic option.
Setup your Cable Modem to run to one Airport Base Station
Setup a computer at the furthest point you can get reception with an wireless card. Setup Internet Connection Sharing for the Ethernet Port and connect your second Base Station to that computer's Ethernet Port. I've only done Internet Connection Sharing a long time ago on a BSD box, but I assume you could do it with Windows, or OS X fairly easily.
Anybody else come up with anything?
Please correct me if I'm way off base.