Note this tip, and the series of tips from 10.3 to 10.8 I've written here all refer to Mac OS X Client. Server versions of Mac OS X may have different limitations, and the people visiting the appropriate Server forum may be able to answer your questions better about Mac OS X Server.
Be sure to backup your data first at least twice before installing any operating system. Shut down, and disconnect any peripherals before continuing with the installation. Read the info below to ensure you are compatible. Finally, you may need to use the Startup Manager to boot the operating system when the 'C' key doesn't work in order to get the installer to work or repair the disk before installation if the initial attempt to install fails. To determine if that repair is necessary, post to the forum, and someone will be able to help you to find out which repairs might be necessary.
Leopard (10.5) is not to be confused with Snow Leopard (10.6) which are different paid operating systems.
Snow leopard is available free for a limited time from this link if you have Mobileme and need an upgrade path to Lion that doesn't require erasing your hard drive.
Flashback malware has a patch on 10.6.8. Users of 10.6.7 and earlier are recommended to disable Java. For more info read this tip.
Macs that were released new as of July 20, 2011, will generally not run Snow Leopard (see the end of this tip for which can and can't). Other than that, the following statements are true:
All Mac Pros will work with Snow Leopard (10.6.x), and they look like:
The PowerMac G5 towers which look like:
will not work with Snow Leopard
All Apple notebooks labelled MacBook with at least 1 GB of RAM, MacBook Pro, and MacBook Air below the screen will work with Snow Leopard.
Apple notebooks labeled iBook, and Powerbook beneath the screen will not work with Snow Leopard.
All iMacs that look like:
Will work with Snow Leopard except the educational iMacs released after July 20, 2011. Those will only work with Lion or Mountain Lion.
Apple has yet to released an publically available iMac that only works with Lion, as of writing of this article.
If they are iMac Intel they can upgraded to Snow Leopard. To tell if they are Intel, they will have an EMC# on the base which is enumerated 2104, 2105, 2110, 2114, 2118, 2111, 2133, or 2134.
Otherwise they are iMac G5, and can't be upgraded to Snow Leopard.
You can also tell if it is an iMac Intel by selecting Apple menu -> About This Mac. Core Duo and Core2Duo are Intel, whereas the G5 are not.
Notes: G5 refers to the CPU made by IBM for Apple before the migration to Intel CPU in 2006. It was found on iMacs, and PowerMacs. Powerbooks and iBooks maxed out using the Motorola G4 CPU, only to be replaced by MacBook Pros and MacBooks in 2006.. Intel made the CPU found in 2006 and newer Macs, and these are referred to as CoreSolo, CoreDuo, Core2Duo, i3, i5, i7, and Xeon. Don't confuse a G5 for an Intel CPU Mac. They are not the same except in exterior design when it comes to the iMac, and the means to tell them apart is stated above. In 2006, the Mac Mini changed from G4 to Intel CoreSolo CPU. In 2006 the iMac changed from G5 to Intel CoreDuo CPU.
All Intel Macs with sufficient RAM older than March 19, 2010 can take the retail 10.6.3 installer disc. All Intel Macs with sufficient RAM older than August 28, 2009 can take the 10.6.0 retail installer disc. This disc must look like and can't say Upgrade, Dropin, or OEM on it.
It is recommended those upgrading from PowerPC follow this tip:
It is recommended you backup your data at least twice before upgrading any software.
It is recommended you check these listings for compatible 10.6 software from:
C!Net, Snow Leopard Wiki,Macintouch, and Apple's listing of compatible printers and scanners
For digital cameras, these RAW formats are supported on 10.6. Note the most recent Mac Mini, iMac, and MacBook Pros
have SD card slots for reading camera media. For all other camera media, Express/34 on 17" MacBook Pro and pre-June 8 2009 MacBook Pros, PCI for Mac Pro, USB, and Firewire card readers exist for all Intel Mac models. Additionally, many multifunction printers have card readers that will work on the Mac. JPEG, TIFF, PNG
are all common formats supported by cameras outside of RAW, though RAW enables you to post process many more features of digital images than the other formats.
To be compatible with the Mac App Store, the Lion updater from the USB Flash drive or App Store, and the Facetime video software in Standard Definition minimum, you'll need the 10.6.6 combo, 10.6.7 combo, 10.6.7 combo with the font update, or the 10.6.8 combo followed by the Thunderbolt update if applicable.
Some security updates are also available, beyond 10.6.8's update.
Sometmies when 10.6 is installed, Rosetta, the application that allows PowerPC programs to run will not automatically install. In those situations, you can manually install Rosetta from the 10.6 installer disc. The following quote explains how to install it manually:
Insert the Mac OS 10.6 installation DVD When the DVD is mounted, select “Optional Installs” then “Optional Installs.mpkg”
Follow the onscreen instructions for agreeing to the software license and selecting the hard drive for installation.
In the “Installation Type” step, select the box next to Rosetta from the list of applications presented. Continue the installation process.
After successful installation, a confirmation message will appear.
These instructions appeared on http://forums.printable.com/showthread.php?t=1110 .
Other User tips of interest in terms of upgrading, include those on upgrading to Lion 10.7:
And Leopard 10.5 (though I would not downgrade to Leopard without erasing your data first):
Which Macs can have Snow Leopard installed, and which can only have Lion installed based on Machine ID (also known as Model Identifier)?
You can find out which gray installer disc came with Macs that can install Snow Leopard newer than March 15, 2010 by reading: http://support.apple.com/kb/ht1159. Machine ID is in Apple menu -> About This Mac -> More info (on 10.7 and later the About Window has System Information instead of More info to access the System Profiler) under the hardware section. The "x" value below can be any number. Older Macs indicated below can use the 10.6.3 retail installer, if not the 10.6 retail installer, if they are older than August 28, 2009. Together with partitioning, the Core2Duo (not CoreDuo, not CoreSolo), Xeon, Core i3, i5, i7 Macs which are Snow Leopard compatible can run both Snow Leopard and Lion, provided they have at least 2 GB of RAM. Partitioning requires an erase of the hard drive. A second internal or external hard drive can boot into a separate operating system on the same Macs. The Macs below which can only run Lion and later, are also known as Lion prebundled Macs. Lion prebundled Macs thankfully can run Windows in virtualization, which would allow them to use the Windows version of software that may only run in Snow Leopard and earlier on Mac OS X. The Macs listed here that won't run Snow Leopard also are not able to use the retail Lion installer USB Flash drive, and must use the instructions on Macworld to create a specialty Flash drive or be cloned onto another hard drive before their prebundled hard drive dies, to be able to restore Lion.
Mac Mini 5,x and later only run Lion and later. Mac Mini 4,x and earlier can run Snow Leopard with at least 1 GB of RAM (that's greater than 768MB of RAM).
MacBook Pro 13" Core 2.4, 2.8 Ghz only run Lion and later, 13" 2.3 & 2.7 can run Snow Leopard, and can 7,x and earlier Machine IDs.
MacBook Pro 15" "Late 2011"* and Retina Display model can only run Lion and later, 15" MacBook Pro "Early 2011" can run Snow Leopard, and can 7,x and earlier Machine IDs.
MacBook Pro 17" 2.4 & 2.5 Ghz can only run Lion and later. 2.2 & 2.3 Ghz can run Snow Leopard and earlier.
Mac Pro 5,1 and earlier can run Snow Leopard.
MacBook with no Air and no Pro on the screen as of 11/30/2011 can all run Snow Leopard with at least 1 GB of RAM.
MacBook Air 4,x and later can't run Snow Leopard, while 3,x and earlier with at least 1 GB of RAM can run Snow Leopard.
iMac 12,1 i3 (EMC 2496 on foot, MC978LL/A) can't run Snow Leopard, while the 12,x i5 and i7 can run Snow Leopard, and the iMac 1,1 through 11,x can run Snow Leopard with at least 1 GB of RAM.
* The release names Early and Late can be gotten by plugging the serial number of the machine in