Multiple Airport Express on a network?

Simple question, simple answer I hope. However, it's not anything I have ever seen explained in simple words for a non techhie like me.

I have a home network based on a year old iMac, an Airport Extreme, an Apple TV box and an Airport Express. The Express allows me to play music upstairs in the house via AirTunes. This has been such a pleasure to be able to do that I now want to port music to the kitchen too. Seems that adding another Airport Express and some inexpensive powered speakers is an economical way to do this, given the price of AirPlay speaker units at present.

Can I put two Airport Express Units on my network? Are there any issues to look out for if I do?

Thanks.

Tom