If all computers are not getting good reception, you can download this free program (inSSIDer). This will show you signals from all WiFi in your area and what channel they are on. You can ensure that you have different channels 1 - 11 you will not always be able to as they can overlap.
You can also place the base station centrally on an upper floor, or atop furniture, because radio waves spread best laterally and down. Reception will be better if the signal does not have to travel at steep angles and if it doesn't have to go through thick walls, mirrors, fish tanks or anything metal.
Another 8 hints from this article on lifehacker.com and more detailed information on this Wikipedia page.