Below in blue is the original prolem and post but it's not as rosy as I thought.
According to the Airport Express manual, I should be able to plug the express into my extreme network and add speakers. I can get everything going but if I want to have both my g5 imacs and the interweb, which is the extreme network connected and still run the speakers from the express station, I cannot do it.
When I try to play the speakers, then my extreme network cuts out. I can only have one or the other.
I'm not sure why this is happening, is there some other firewall stuff that I need to do. In the express manual it just says to plug it in and select it from the menu, but that disconnects my g5 network.
What I want to do is play some itunes that is on the other g5 through the speakers, so I need my g5 network and the speakers plugged in via express. I'm sure this must be possible.
Or am I missing something here?
OK I've been perusing the apple support pages and I can see that my express has to be a "client" or in "client mode" to just work as an Airtunes access point, but I can't see how to do this.
Actually this is getting to be a bit of a @$%#ing pain. I opened Airport again and I could see my Extreme network and Express there. OK I clicked on Express and it asked if I wanted to switch networks I said OK because I wanted to see what options were there. It asked if I wanted to have it join an existing network as a wireless client. That seemed perfect, then it asked for the network I wanted it to join and selected my extreme network. That must be it. It then said express was restarting which just kept going for ages, but my Extreme network had vanished but I thought that was just because I was setting up the express. I closed the window because it said I could but when I reopened aiport, there was no extreme network anymore. WTF.
OK I've got two macs on an Airport extreme network with a router plugged in and it's all working fine.
I'm trying to set up some speaker in another room with the Express station. Anyhoo before reading anything I just dived in and buggered my Extreme connection. OK that's all reset up now and running fine.
I've looked at the airport instructions for connecting to an existing network which is what I want to do. I can see the express card when I open Airport utility, and i can see the default 6 digit hex name for it appear in the menu.
The instructions just say to plug in the speakers and then turn it on and to join an existing network I just select it from the menu, but when I do that it turns off my existing network.
I went to Airport utility to see if I needed to set it up but when I try to that also wants to try and make a new network which I don't want. I just want it to connect to my existing extreme network to play the speaker. I can see the express on my menu so I know it's being read.
But even on my iTunes I cannot see a button that is supposed to be on the bottom right corner of the iTunes window for remote speaker. I have "look for remote speakers connected with Airtunes" ticked in iTunes prefs.
So that's about it, I can't see what I've missed. I've got a feeling it's got something to do with the button that is not there in my iTunes window.
any assistance, as usual, is appreciated.
EDIT: If I select the the express base station in the menu the Speaker button comes on and I can control the speaker. Yay. But it then cuts off my network. 
EDIT: after switching between networks a few times suddenly the button is staying there and the network remains. Phew, what a scorcher.