Q: Which is the latest OS x i can use ?

Yes, Leopard can run quite as well as Tiger on your Powerbook.

Hi...

See if your Mac meets the Requirements for Mac OS X v10.5

Make sure there's enough free disk space ..

Right or control click the MacintoshHD icon. Click Get Info. In the Get Info window you will see Capacity and Available. Make sure there's a minimum of 15% free disk space.

My PowerBookG4 runs a little slower on Leopard then it did on Tiger and I keep the as much free storage space available as I can.

You can run Leopard which is also the last operating system you can get since you have PowerPC -based computer (G3, G4 and G5 are all PPC). Leopard is slower than Tiger, I have a PowerBook with 1GHz G4 and it does not matter if I run it with 512MB or 1.25GB memory, Leopard feels sluggish compared to Tiger. I think Apple was far more focused on Intel version than PPC at that time which is easy to understand.

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

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

When I was looking for a 10.5 upgrade disk it was so ridiculuosly expensive, they were all almost $200 where other OS versions were less than half.  Eventually I was able to find one, but had a lot of post upgrade issues that made it necessary to have the 10.5.8 upgrade available too.  Make sure you download that package before you run the 10.5 upgrade disk!

If you wish to get the best performance then upgrade the RAM to 2 GB (1GB in each slot). This is a do it yourself operation and iFixit has good instructions.

http://www.ifixit.com/Guide/Installing-PowerBook-G4-Aluminum-15-Inch-1-1-5-GHz-R AM/220/1