I find I have software that needs 10.5, how should I upgrade?

Question

10.5 is needed for many new iApp based devices, yet Apple no longer offers it on their web based store.  For people who have compatible computers they need to consider the following:
1. iTunes latest version that works on 10.4.11 is 9.2.1, available here: http://support.apple.com/kb/dl1056
2. Boot Camp only works on Intel Macs, not PowerPC G4 nor G5 Macs.
3. Classic does not operate in 10.5.  This user tip does show what Macs allow dual booting of 9 and 10.5:
https://discussions.apple.com/docs/DOC-1757
Otherwise you'll need to keep 10.4.11 or earlier to allow Classic compatibility either on an erase and reinstalled 10.4 partition, as described in the tip, and
http://docs.info.apple.com/article.html?artnum=61301
or an external Firewire hard drive.  If you have an Intel Mac, this does not matter since it doesn't have any 9 compatibility at all.
4. Apple may offer the retail version of 10.5 through its phone mail order, but not show it on their website.  Note this is the only version of 10.5 that will run on any Mac that shipped new prior to October 26, 2007.  System specific discs rarely if ever work on Macs that aren't the same vintage or model they were initially designed for.  And Up to Date discs were only available for Macs that were new October 8, 2007 to October 26, 2007.
5. Macs need 512MB of RAM, a G4 processor with 867 Mhz (dual 800 Mhz doesn't count, and upgraded processors only count if the processor seller supports it), and a minimum of 9 GB of free hard disk space.  Arbitrarily an additional 15% free hard disk space has been found in addition to Apple's specs to suffice for fast smooth installations of Mac OS X.  I.e. 100 GB hard drive would have at least 24 GB free before their 10.5 installations are smooth.
6. All upgrades necessitate backing up your data at least twice, checking for software and hardware compatibility, and shutting down the Mac, and disconnecting all but display, Apple USB keyboard/mouse (if desktop), before rebooting and installing the update.
7. If your Mac is Intel, upgrading to 10.6 (Snow Leopard) directly is recommended, and if you have a Core2Duo or Xeon Mac, upgrading to 10.6, then 10.6.6 or later to get Lion (10.7) installed is recommended if you have software which supports it. Contrary to popular belief 10.5 is not needed as an in between installation to apply 10.6.  But 10.6.6, 10.6.7, or 10.6.8 presently are a minimum for installing 10.7.  10.7 no longer supports Rosetta though. So be prepared if you are in the market for it.  Snow Leopard requires 5 GB of hard disk space and 1 GB of RAM.  Lion requires 7 GB of hard disk space and 2 GB of RAM.  The same arbitrary 15% as mentioned in #5 is highly recommended for free hard disk space that is additional.  Be sure to inquire in the Communities before updating anything to find out about any additional pitfalls you may run into.

Answer 1

"this tip is ready for consideration"