Q: Help with Airport Express as access point

I have the Airport Extreeme that I use for my wireless router. The range on it was not able to reach the far parts of the house, so I was told by an Apple store trep that I could purchase and use an Airport Express as a wireless access point to extend the range of my network.  So...I bought one.  Now...how can I configure it so that it simpy extends the range?  I don't want it to change IP addresses for my computers, as that is a problem I ran into with a past access point....then I would have two computers with the same IP address and it made for all sorts of problems.  All I want the Airport Express to do is is extend my range.  Help?