you might want to experiment.
go to
www.netstumbler and download it. once you have it installed on the
machine upstairs start fiddling about with your antenna's (antenne?) slowly
while on the phone to the guy upstairs until you get the best signal.
net stumbler is a free download mostly used for network sniffing /
wardriving, but it has many legitimate uses too, such as exactly what you
are wanting to do.
tbh, if it's just upstairs you probably won't have any trouble anyway, just
try and keep it as close to where the wifi card is upstairs and you should
be fine.
hope this helps.