You have been getting stellar advice from two of the most knowledgeable, experienced and helpful contributors. I'd never presume to be in their league, and both of them have helped me tremendously over the years.
On the other hand, I'd like to ask you a single question: Do you really need Leopard 10.5.x at all?
The reason for that is that I have dual boot partitions for 10.4.11 and for 10.5.8, but I find myself running 10.4.11 exclusively, simply because Tiger 10.4.11 beats the pants off Leopard 10.5.8 on my g5 Quad performance-wise. I also have an Epson Expression scanner that runs just as well under Leopard as it runs under Tiger. It's just that Tiger is definitely snappier. I kept Leopard for "just in case", but I've never needed it.
Below are my machine's specs:
2.5 GHz Power Mac (PPC) G5-Quad; 16GB RAM; mutant, flashed 550MHz nVidia GeForce 7800GTX 1,700MHz 512MB VRAM; ATTO ExpressPCI UL5D LP SCSI card; Mac OS X Tiger 10.4.11 and Leopard 10.5.8 boot drives; Spotblight, Dashboard and Time Machine permanently disabled; dual 22" CRT monitors; USB wireless 'n' available but connected to the Internet via wired Ethernet; FW flatbed scanner; 2 SCSI scanners (one tabloid-size transparency scanner [Epson Expression 836XL] and a film scanner); various internal & external HDs; FW Epson 2200 and Ethernet Samsung ML-2850ND printers; 2 X Back-UPS RS 1500 XS units.
VueScan version 9.0.86, which I'll upade to 9.0.94 in a few minutes.
You should also read the latest Release Notes on the VueScan page, just in case you want to make your scanner work inLeopard:
If you're using a Ratoc Firewire-SCSI adapter, you may need to configure one of the options. Try setting the time between the power on and the SCSI bus reset to 40.
If you're using the Ratoc USB->SCSI adapter, set up the adapter with the Ratoc utility to "Mass Storage" mode.
If you're using a SCSI scanner on Mac OS X 10.4 (Tiger), have an Adaptec 2906 or 2930CU SCSI controller and your SCSI scanner isn't working properly, try the following commands in the terminal window:
- cd /System/Library/Extensions
- sudo mv Adaptec290X-2930.kext Adaptec290X-2930.kext.sav
- reboot the computer
To restore this change:
• cd /System/Library/Extensions
• sudo mv Adaptec290X-2930.kext.sav Adaptec290X-2930.kext
• reboot the computer
If VueScan still doesn't see your scanner, go into "System Preferences", "Accounts", then "Startup Items". Make sure nothing is being started that might be accessing the scanner (i.e. "Epson Scanner Monitor").
If you have a SCSI scanner but don't have a SCSI adapter, try using the Ratoc Firewire->SCSI adapter.
If your bootable backup of 10.4.11 has everything you need on it, including all data, overwriting the hard disk with the backup will be a trivial task, easy and straightforward.
If not, then precautions will have to be taken to preserve what you need.
Personally I've never had occasion to downgrade to an earlier OS, I've always upgraded. When I installed Leopard, I did so on a virgin, blank partition of a physically separate internal hard disk. I tested it extensively and eventually found myself using Tiger, The Leopard boot partition remains there "just in case." I've never needed it, though. I have Snow Leopard 10.6.8 on a MacBook.
BDAqua already encouraged you to tell us more about your backup. I second that suggestion.