Air Port Express

I have a Time Capsule 500GB, dual band 802,11n Aiport Extreme base station. Yet, in some parts of the apartment, my wifi signal is too low. I have been told that if I get an Airport Express Base St. I could extend the power of my wifi signal.

1. buying this Airport Express will solve this problem, and will not interfere with my Time Capsule??

2. If I have to configuer it, should I choose different passwords than the ones I have in my Time Capsule?

Thank you!

Lucila