^ha! @wiggles - there ain't nothing you can't fix with some good ol duct tape :~P
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@baller - try using Xirrus on your wireless computers.
Xirrus is free and it will help you to troubleshoot connections, range, and them nasty rouge APz

I've had assignments in the past in which I had to use repeaters or even use two wireless routers to address the range issue.

I say start with the basics: position position position

What are your options? See if you are able to reposition the modem/wifi router at a more central location, having it next to a farside wall is real 'derp'
Why have half of your signal 'bubble' hang outside of the house AND weakened by the wall??

also make certain your wireless router is not near any metal and other electronic devices such as the home phone, TV, even the modem.
Though you are trying to get signal to the lower level, make sure the router is not on the floor yet not up on a high shelf.

check the antennae, is it omnidirectional? you could replace it with a high-gain antenna, aim it towards the lower level and it may do the trick +

Also try changing the wireless channel on the router, run through them all or find out which ones are recommended in your area.

if none of this helps, a repeater would make perfect sense from this point, I would recommend one made by the same company as your router. ^,^