Hello and welcome to my User Tip.
OS X 10.4 "Tiger" and 10.5 "Leopard" is obsolete and insecure, Apple is not paying attention to those versions of OS X anymore.
You need to purchase the 10.6 disk from Apple via the online store to upgrade OS X to 10.6.8
10.6 has Rosetta which allows PPC code of 10.5 programs to continue to run in 10.6 which is now 100% Intel processor based code.
Only Intel processor Mac's can upgrade to 10.6 and only some Intel Mac's can upgrade further, most 10.4/10.5 from the factory Mac's should remain on 10.6.8 due to age/performance slowdown and many new program purchases, a newer machine is a better option.
10.4 users can skip 10.5 and go directly to 10.6.3 (+ Software Update to 10.6.8) using the disk.
Once upgraded to 10.6, it's very unlikely the firmware will allow booting from the previous 10.4/10.5 disks.
First you need to clone your present 10.5 system to a external powered drive formatted OS X Extended (default) in Disk Utility using Carbon Copy Cloner (version for 10.5) then disconnect. If you have a Firewire drive use that, as I recall older Mac's can boot off Firefire only, not USB.
Next using another external drive, copy just copy the contents of your home folders (music, movies, pictures etc) to it and disconnect. This is your files backup in addition to the full clone as you need to have two copies of your personal files maintained on separate hardware at all times. (something bad might happen to the boot drive during the upgrade)
Temporarily reduce users files for performance for boot hard drives
Next what you want to do is reduce the amount of files in your Home folders of Music, Pictures etc (not Library)., that you just copied to the external drive, to the Trash Can and make sure to Empty Trash. The more the better and thus reduce the system to only OS X and installed programs.
Upgrade to 10.6 in place
Next you insert the 10.6 disk inside and there is a installer, run that and it should (barring other issues) should reboot and upgrade the system to 10.6 Once it does, head to Apple Menu > Software Update and do that until it's complete.
All your programs, users etc., should be left intact, but things might have been upgraded in other areas like your iLife suite and Apple supplied programs.
Return users files
Next connect the storage drive and return those files into their appropriate Music, Pictures, Movies etc., folders and open Activity Monitor and make sure your preferably below 50% of the hard drive filled for best performance, but not more that 80% filled.
You might decide to keep some large files like Movies or Pictures on the external drive, as they are large and not as often viewed, so they make a good choice for a external solution. iTunes music likely should be left on the main boot drive as to be able to play music whenever you need to.
Doing so like I outline will not cause the machine to be slow when upgrading on hard drives, as they slow down the computer the more they are filled up. Also your 10.5 programs should work, but may require a update for some.
If the upgrade install goes badly
You might have to do a erase and fresh install of 10.6, which the 10.6 disks also do if hold c or option/alt key booted from them, but in doing so you will have to likely have to erase all programs and installed (but backed up!) files and start with a brand new system. Install 10.6 compatible versions of your software.
Second option is to hold option/alt (wired or built in keyboard) and boot off the 10.5 clone drive you made earlier and either reformat the entire internal drive in 10.5's Disk Utility (a must to reset the drive back to a compatible format) and reverse clone 10.5 back on with everything and be back to square one.
Or manually transfer those self contained programs (everything is in the icon/package in your Applications folder) and move files back into the new user account created on the now 10.6.8 freshly installed machine.
There is a MacintoshHD/Users/NewUser/Public/DropBox folder which you can copy the files too and the permissions and ownership will change to the new user account from the old one.
Think twice about upgrading OS X further than 10.6.8
At this point you may think you should further upgrade to 10.7 or 10.8, however this the problem, your 10.4/10.5 factory machine is dated. It might not run the later OS X versions and even if it could, they will be slow as they are designed for more powerful later processors with more features and more powerful graphics. Also your PPC software will not work on those later OS X versions, there is also newer hardware based features that you can't take advantage of with a older machine.
At this point you should consider your options in purchasing a newer machine rather than wasting time and money buying all new software as well for a older machine. The older machine can be used as backup or later sold.
You already upgraded to 10.7/10.8 and now want to return to 10.6.8
You want to hang onto 10.6.8 as long as you can
This is the estimated OS X version market share as of the date of this posting, June 1, 2013 based upon Net Applications data.
As you can see OS X 10.6 Snow Leopard has a significant chunk of the market, mainly due to third party software that hasn't yet made itself, or doesn't want to be, part of the AppStore environment that was introduced in 10.6.8 and later OS X versions.
In 10.8 Apple introduced Gatekeeper as a security feature, it's default settings only allow software from Apple's AppStore and Apple ID'ed developers (can presently be overridden for now with a control/right click), which naturally will also place their programs on AppStore.
So it appears the stage is being set for a iOS like closed operating system for future Mac's much like it is on iOS devices. In fact OS X has been adding more iOS like features at each revision, so that's a warning that being able to install software from outside sources may be inhibited in future OS X versions.
OS X 10.6 gives the maximum software flexibility in this area as it can run software from outside sources, versions that are now only on AppStore and also PPC programs via Rosetta. Also it's the fastest OS X version currently supported for security updates freshly installed on the same hardware.
As developers recode their software for the very possible future closed machine and AppStore restrictive requirements, you may wish to stick with 10.6 as long as possible or seek relief in Windows 7 either directly installed in BootCamp or via virtual machine software in Snow Leopard. Windows 7 will remain as is and supported until 2020, since many new PC's come with Windows 8 (yuck) if you get a Windows 8 Pro machine (it has to be Win 8 Pro+) it then can be downgraded to Windows 7.