Q: I need to upgrade system to a powerbook G4. no Intel processor... so which is the latest system I can have installed on this computer? Thanks

With a G4 Leopard is it, assuming your computer even meets Leopard's system requirements:

Leopard 10.5 Technical Requirements - http://support.apple.com/kb/SP517

Get more information about your computer. Go to the Apple in the upper left corner of any window, then "About This Mac", then "More Info..."  Copy and paste the information here, but omit the serial number and Hardware UUID (if present).

Also of interest:

List of Applications Not Compatible with Leopard - http://guides.macrumors.com/List:Applications_Not_Compatible_with_Leopard

Leopard is not easy to find, nor cheap.

Texas Mac Man, Leopard disc sources - https://discussions.apple.com/thread/3636662

"a brody" post about upgrading to Leopard - https://discussions.apple.com/thread/3373795

After all this if you are anticipating having to spend several hundred dollars to upgrade you might just consider jumping to a used Intel machine where you can get Snow Leopard for $29 if it doesn't already come with it.

Neville Hillyer wrote:

Those wishing to upgrade to Leopard should be aware that install disks can be expensive unless you contact Apple.

But Apple doesn't  stock retail Leopard any longer.  I don't think you can even talk them out of a copy the way you could a few years back.  The only sources are third party and yes, they will charge what the market will bear.

Important

Check that your Mac complies with any requirements. If you are not in the US you should use the Change Country link at the bottom of Apple pages.

If you're talking abut system requirements, yes.  However, hardware requirements are going to be the same anywhere.

My understanding was that Apple will still supply a copy of an original lost disk for a nominal fee after checking the Mac's serial number. I assume these disks may be model specific although other sites imply they may not be - perhaps my wording on this point needs to be more circumspect.

Apple always used to keep a stock of non-model specific disks for volume license holders who were promised further disks for a nominal handling fee. For an example a public body may have Leopard limited Macs and wish to supplement their 1,000 off license originally delivered with one disk. I don't recall Apple ever saying there was a time limit on this service.

The 'important' notes were intended to be separate issues - I will change this.

Revised version below:

Yes, they do keep a limited backstock of model specific discs.  Apple does consider anything older than 5 years as vintage and has discontinued any hardware support which could likely include accompanying installer discs unless they find a dusty set in the bottom of a drawer.

  Vintage and obsolete products - http://support.apple.com/kb/HT1752

The big "however" in all this is the original post was about upgrading a G4 to Leopard, and one which required a retail Leopard discs.  When Snow Leopard first came out Apple stopped selling Tiger.  Still, you could still go to Apple and cry on their shoulder that you had a G4 that wasn't rated for Leopard and could they pretty please find you a set of retail Tiger discs for sale.  That lasted a bit, then people started reporting back they couldn't find Tiger at Apple for neither love nor money.   I can't speak for Apple but given that even Snow Leopard is but hours away from becoming third generation, I very much doubt they still have any retail copies of a system they stopped selling in the stores 3 years ago, especially one that was so heavily in demand.  I read reports by posters of seeing Leopard selling for $150+ which suggests to me there isn't an old Apple retail price source left around.

Leopard is no longer available at the Apple Store but may be available by calling Apple Phone Sales @ 1-800-MY-APPLE (1-800-692-7753).

If you can't obtain a retail install DVD from Apple, look on eBay or Google the installer part numbers to possibly find at an on-line store. Here's what to look for:

MB427Z/A  Leopard 10.5.1 install DVD

MB576Z/A  Leopard 10.5.4 install DVD

MB021Z/A  Leopard 10.5.6 install DVD (single user)

MB022Z/A  Leopard 10.5.6 install DVD (5-user family pack)

Installing Mac OS X 10.5 Leopard

http://support.apple.com/kb/HT1544

Mac OS X 10.5 Leopard Installation and Setup Guide

http://manuals.info.apple.com/en/leopard_install-setup.pdf

After you install the base 10.5, download & install the 10.5.8 combo update at http://support.apple.com/downloads/Mac_OS_X_10_5_8_Combo_Update

The DVD should look like this

Caution - Leopard does not support classic mode. So, if you currently open OS 9 apps in classic mode, you won't be able to do this if you upgrade to Leopard.

 Cheers, Tom

Leopard does not support classic mode. So, if you currently open OS 9 apps in classic mode, you won't be able to do this if you upgrade to Leopard.

My Leopard supports booting into OS 9.2.2 in many locations including the same partition as Leopard. My OS 9 supports booting into Leopard but it ignores all OS X security. Selecting OS via option at boot requires separate partitions.

"Classic" isn't the same as native booting to OS9.  You are right about the G4 likely booting natively to OS9, TMM is right about OS9 as Classic not being available in Leopard.