First two things to look at, are the router & the wifi card, not the Internet connection. You are better in using a A/P upstairs if the signal is not best from the router. As for the antenna on that card, if the PC is shoved into a cabinet, the metal of the chassis for the PC can cause problems. Another thing, run InSSIDer from metageek.net, to see what the SQ is from your router, and if there are any other devices interfering with your signal.
Personally, there is too much junk out there badged as Internet devices.