Hello,
I have a G4 powermac AGP in my studio and want to get a G4 iMac to put in the house. I want both of these computers to run off a wireless network - namely with Apple Airport Extreme Base Station. I understand that I can only fit an 'old style' Airport card to the Powermac which is fine as there is no option, but I'm told that some G4 iMacs will take the newer Airport Extreme card. Does anyone know what the LOWEST spec (cheapest!!) G4 iMac is compatible with AirPort Extreme??

You should note that when you have mixed (802.11b & g) groupings like this, the wifi router will run at the slowest speed.

That being said:

The 17" 1GHz G4 iMac was the "first", after that, the 15" 1GHz G4 iMac (with USB 2.0).. and so on.

Does that means I'd be better off getting an AirPort Express Base Station? Does that only run at the slower speed? I know I'd find AirTunes useful..........

IMO, there's no reason in the world to get an Apple wireless router. They are massively overpriced and your AirPort cards will work with pretty much any wireless router out there. And you can get them a lot cheaper. ANy wireless router that supports both 802.11b & g will run at the slowest speed available when there's a slower card (802.11b) connected to it. That's just the way it works.

Ooops!! I've literally just bought an Extreme Base Station off ebay. Got it for a fairly decent price though. I've been told that I need a separate router or at least a modem with a built in router. Is this correct??
If it is, do you know how to connect them?

Well, you will need a separate cable/DSL modem.. but not a separate router. There should be instructions that come with it, if not, hit the Apple support website. But typically it'll use an ethernet cable (Cat-5 or cat-6) to connect your router and your broadband modem.

Internet <-> DSL/Cable Modem <-> Wifi Router <-> home computers

Do you know what the main differences are between the Extreme and the Express??

Seem like not a lot. Express comes with AirTunes which seems great, while the Extreme looks very pretty but only comes with a dial-up modem. Doesn't seem to be a great deal of difference between the two.............except the price!!