Q: What's the latest OS I can put on a G4?

Mac OS X 10.5. If the computer's CPU speed is less than 867MHz, Mac OS X 10.5 won't install normally, but will boot and function if an installation created elsewhere is moved to it or done through FireWire Target Disk mode.

Requirements for Mac OS X 10.5

https://discussions.apple.com/message/10044889#10044889

Parker has you well covered, just to perhaps clatify...

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)

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

There are workarounds if the 867MHz is the only hangup...

http://sourceforge.net/projects/leopardassist/

List of Applications Not Compatible with Leopard...

http://guides.macrumors.com/List:Applications_Not_Compatible_with_Leopard

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

Irun it on a G4/450 MHz at reasonable but not fast speeds, it depends more on the Fraphi card with 10.5.x, as it runs better on the G4/450 with a128 MB Graphic card ta it does on an eMac 800 MHz with a lowly 32 MVB Graphic card.

At the Apple Icon at top left>About this Mac.

Then click on More Info>Hardware>Graphics/Displays and report like this...

NVIDIA GeForce 7800GT:

  Chipset Model:    GeForce 7800GT

  Type:    Display

  Bus:    PCI

  Slot:    SLOT-1

  VRAM (Total):    256 MB

  Vendor:    nVIDIA (0x10de)

  Device ID:    0x0092

  Revision ID:    0x00a1

  ROM Revision:    2152.2

  Displays:

VGA Display:

  Resolution:    1920 x 1080 @ 60 Hz

  Depth:    32-bit Color

  Core Image:    Supported

  Main Display:    Yes

  Mirror:    Off

  Online:    Yes

  Quartz Extreme:    Supported

Display:

  Status:    No display connected

So we know more about it...

At the Apple Icon at top left>About this Mac, then click on More Info, then click on Hardware> and report this upto but not including the Serial#...

Hardware Overview:

Model Name: eMac

Model Identifier: PowerMac6,4

Processor Name: PowerPC G4 (1.2)

Processor Speed: 1.42 GHz

Number Of CPUs: 1

L2 Cache (per CPU): 512 KB

Memory: 2 GB

Bus Speed: 167 MHz

Boot ROM Version: 4.9.2f1