Aftab threw up a thread some time back about the Alpha brand of WIFI estensions like this one:
Alfa Super Wi Fi Extender 1000mW 802 11b G USB WiFi Network Adapter | eBay
I have one of these that took me from "0" signal to 5 bars through 5 block walls. Definitely worth looking into as well if you have signal problems.
Bear in mind the wifi extender listed here only supports the older (read slower) "b" and "g" wifi standards.