You can also buy a Linksys, Buffalo, or ASUS router and flash it with a DD-WRT firmware, and if you don't mind buying two, you can create a WDS network with your wireless routers using the custom firmware. One router becomes the primary connection, and the other router grabs the signal and repeats it. This is a better signal then using a wifi booster.

You can try removing one of the ants. (if the router has two) and replacing it with a higher DBi replacement that is aiming upstairs.

A Wireless repeater from Linksys would also be a good choice, these usually just plug into a wall socket and repeat the signal.