I'm thinking of getting an iMac 24" for our main room, that would store all of our music on.

Then we would have 4/5 airtunes connected to sets of speakers in different rooms in our house.

Then the iPod touch, that would act as the remote, controlling all our music playback throughout the house.

Can this be done with remote buddy? 
Can music be played to more than one airtunes box at the same time? 
Would I need to type in an IP Address, username & password every time I came in an picked up the remote?

Thanks for advance for your answers. 

I want to do exactly the same thing. Don't have an iPod touch yet but am using the Remote Buddy AJAX browser on a laptop to "simulate" an iPhone (until they arrive in France).

So far, it all looks great. I can switch between Airport Express units, control iTunes, select playlists, etc. etc.

I would also recommend buying "Airfoil" from Rogue Amoeba. With that, you can stream anything to your AXs (not just iTunes). I wrote an Applescript that I can launch from within Remote Buddy that:

1) Opens Airfoil 
2) Opens Realplayer 
3) Opens the BBC Radio 1 stream (or whatever stream you want) 
4) Sends it to a selected Airport Express

Can't recommend Remote Buddy highly enough if you want to try and do multi-room music.

Good luck. 

Does remote buddy allow you to connect to more than one airtunes at a time? So I could play music in multiple rooms at once.

Does the address remain static so you can bookmark the link so you can access remote buddy quickly? 

The address does remain static so it is easy to get to. I would suggest downloading a demo copy of RB and giving it a try. Some parts are a bit confusing (to me at least) but it looks very powerful. 

Regarding the address and login: 
- You can turn off the login mask and password protection in the 
preferences, although I would not recommend this for security reasons.

- If you have set up your network in a fashion that Bonjour can work 
and/or that the IP of your computer stays the same, you can just add 
the AJAX Remote to your bookmarks.

- You can log in once and stay logged in for as long as neither Remote 
Buddy and/or your Mac get restarted.

I, too, strongly encourage you to just give the demo a try. It's 
feature unlimited and will allow you to explore it to your hearts 
content. Also the FAQ will provide you with solutions should you 
encounter any issues during setup.

Best regards, 
Felix