can 10.5.8 be installed on a Powerbook g4 1.5ghz with 1.25 gig of ram

Yes. Any retail DVD can be used. Disks which originally shipped with a Mac can't be.

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I bought the Leopard 10.5.6 Box Set (includes iLife and iWork) and installed it on my 12" 1.33GHz with 1.25GB RAM (which originally came with 10.3.x). It runs flawlessly!

Yes...

Leopard requirements/10.5.x...

* Mac computer with an Intel, PowerPC G5, or PowerPC G4 (867MHz or faster) processor

minimum system requirements

* 512MB of memory (I say 1.5GB for PPC at least, 2-3GB minimum for IntelMacs)

* DVD drive for installation

* 9GB of available disk space (I say 30GB at least)

http://www.ebay.com/sch/i.html?_nkw=mac+os+x+leopard+retail+10.5

Mac OS X 10.5 Leopard installation system requirements

http://support.apple.com/kb/TA24950

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

If you are currently using OS 9 apps in classic mode, be advised that Leopard doesn't run classic mode.

 Cheers, Tom 😉