Q: I have a Power PC G4 (1.5 Ghz) running OS X 10.4.11.  Can I upgrade to OS X 10.5?  What would be the downside of an upgrade?

Yes.

Mac OS X 10.5 doesn't support Classic.

(65483)

Besides no Classic, 10.5 requires twice the RAM, and a better Video card than you can get away with in 10.4

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)

Classic/OS9 Apps no longer supported.

Trouble is Apple no longer sells it, check eBay & such for the Retail version, not the Gray Discs...

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

No classic operation, you may need more RAM and the cost of a retail Leopard 10.5 install DVD; and it may run a bit slower than Tiger. Getting a copy of Leopard.

Adavntages - You will be able to sync an iPad, iPod or iPhone. You can use a newer version browser.

BTW - Leopard is the max version OS that will run on a PowerPC processor Mac, like yours.

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

 Cheers, Tom