This might be a little confusing to put into words, but here it goes.
We have a DSL Modem/Wifi router downstairs, the upstairs doesn't get coverage. What my solution has been is to put my desktop near the top of the stairs where I get a signal, bridge that connection, run an ethernet cable from my computer to a second router (an old Belkin G router I had lying around) and then use that as a source for wifi upstairs.
Recently though, I noticed that my Xbox (which is upstairs) doesn't see my computer. Its really annoying.
UPDATE: So I took my Xbox downstairs, and it still can't see my computer, and I messed with the downstairs router so that it doesn't reserve IP addresses anymore. Which helped out with the internet connectivity issues. I configured the upstairs router to no longer handle DHCP.
So I got it working. I tried lots of different solutions. I eventually set the Belkin upstairs to be an access point and gave it the same IP and DNS as the computer it was connected to. That worked for a while, then I started getting IP address conflicts, so I tried to mess with the router again and I couldn't get the Admin page. So I reset the Belkin router and now its just working. Like magic. So when in doubt, reset that thing a few times and pray.