Thanks, yes that's clear, though I'm afraid the answer won't be 🙂
Your original AirPort Express can be configured with your PowerBook G4 just fine, but it's too old to be configured with your newer MacBook Pro without resorting to a somewhat inconvenient workaround. Therefore, use your PowerBook to configure the Express, should that be necessary. It's not necessary if your network is already functioning though.
I didn't go far back enough in the links I posted regarding Mac versions of AirPort Utility. For your G4, download and install AirPort Utility 5.5.3. That should work for your Express, and there will be no pressing need to purchase a new AirPort Base Station.
If you decide to buy a new Express or Extreme though, you will need to configure it with your newer MacBook Pro or one of your iOS devices. The PowerBook will connect to its wireless network without any problem, but it can run no version of AirPort Utility capable of configuring any current production AirPort Base Station.
As far as AirPrint is concerned, your HP Photosmart 7520 is an AirPrint - enabled printer, but the 3310 is not. If you require that printer to be available for AirPrint on your iOS devices, the most cost-effective solution for you might be this dedicated AirPrint print server from Lantronix:
http://www.lantronix.com/it-management/xprintserver/xprintserver.html
There are workarounds that can enable your MacBook Pro to act as an AirPrint print server, but the disadvantage would be that the MacBook Pro would need to be connected to power and have its lid open for it to be available for printing purposes. Research the following two products:
handyPrint (donation-supported)
Printopia ($19.95 with free trial)
Your wireless router - in your case, the Express - is an essential link in the AirPrint communication chain. Off the top of my head I am not certain if your original Express will support the protocol required for AirPrint but I believe it does. I no longer have one that I can test.