I actually read somewhere that distance is more of a problem for wireless routers (2.4GHz) than walls, floors, furniture, etc. They are, of course, talking distances of 50~100 feet, so you should be ok if the article I read was correct

I agree with Bailey that 20feet and even a couple outside walls you should still be able to connect. I would put the router by the computer used most often, or make it so that it gets the strongest signal. Worst case scenario, maybe the upstairs computer only connects at 20~30Mbps instead of 54