Wireless Router and Wireless Access Point setup

So, I am looking for recommendations on what would work best. If you are using a Linksys Wireless Router in the basement and setting up a Linksys Wireless Access Point on the second floor. Would you set both of them up using the same exact SSID and Encryption Key, or would you give them separate SSID's? I've done it both ways, but never really played around with it enough to see what works better. Does it even matter, or would a wireless laptop just connect to whichever is broadcasting stronger given that you don't set either of them as a preferred network?

This will be used for a Windows XP machine.

Thanks. I actually lucked out and was able to re-configure the customer's laptop and router to work better than whoever originally installed it. So he is still using just one router with full coverage.