First off, I wanted to say thanks to everyone who helped me on this board. And a huge thanks to LTL whose guide really helped me out and is the basis for this post.
http://www.insanelymac.com/forum/index.php?showtopic=86167

So I thought I'd post my steps for a retail install of Leopard on a system with a Gigabyte GA-G31M-ESL2 board. You will need a Leopard install to make this work. I used XxX 10.5.6 PPF3 that was already installed on another drive in my system.

Step 1. Start Disk Utility and select the drive you want to install. Click partition. I usually select 2 partitions since I don't plan to use the whole disk for Leopard. Change the size of the first partition to the size you want Leopard to be. I chose 30 GB. Select Mac OS X Extended (Journaled) for the first partition, and Free Space for the second. You can choose a name for the first partition. I used Leopard.

Step 2. Insert the Leopard DVD or mount the DVD image.

Step 3. Start Terminal and enter the following:

open /Volumes/Mac OS X Install DVD/System/Installation/Packages/OSInstall.mpkg

Run the installer and select the right drive. I removed the printer drivers, additional fonts, and language translations.

edit: Step 3a (Optional). Install any Mac OS X System Updates. If you have a 10.5.6 Retail disc, you can install the 10.5.7 delta update now. Apple 10.5.7 Delta Update You should be able to use the Apple 10.5.7 Combo Update if you have an earlier Retail disc, but I haven't tested this. Make sure you select the right drive when installing this.

Step 4. Download leopard-install-kit.zip. It's basically a modified version of LTL's v4.4 files. It includes drivers and EFI strings specific to this motherboard and a newer version of EFI Studio so you can add your graphic card strings later.

Step 5. Open leopard-install-kit.zip and edit the post-patch.sh script inside with TextEdit or whatever text editor you like (I prefer jEdit). Near the top of the script is a line that says DISK="Leopard". You need to change this to your disk's name. No other changes should be necessary.

Step 6. Run post-patch from Terminal.

sudo sh ./post-patch.sh

The script will check to see if it's root and won't run otherwise.

Step 7. Install pc-efi v9 from the netkas folder inside leopard-install-kit. Make sure to select the right disk.

When you reboot into Leopard, you will need to go into System Preferences -> Sound, and select Line Out for the sound. I don't have any nice speaker setup, so I don't know what else works. I can play video off YouTube, MP3/AAC files in iTunes, and hear all the sound effects Leopard makes.

EFI Studio and About This Mac are in the Applications folder as zip files. You can use EFI Studio to add your graphics strings (don't forget you also need to add AZAL and Ethernet because it all has to be done together - neither Sound nor Ethernet work properly without EFI strings). You can use About This Mac to fix that screen to show your correct processor. It's a cosmetic change only, so it's not really necessary.



http://img17.imageshack.us/img17/5317/aboutthismac.gif

Enjoy. Please let me know if something isn't working.

edit: I just tried Software Update for the first time. Things not requiring restart seem to work fine. I updated Java easily. Things that require a restart are more tricky. It went to this blank blue screen that had the Apple rainbow pinwheel for about 25 minutes. When it finally restarted, it came up with no desktop icons and the menu was frozen. I had to use the reset button on the case. When it came back, everything is fine and the update no longer appears in the Software Update list, so it seems to have worked. However, the first time I tried it I thought the pinwheel was Apple BSOD and I hit the restart too soon and borked my install. The other restart required updates QuickTime, iLife, and AirPort Client don't interest me and I don't feel brave enough to tempt fate right now. I ran verify disk after I had to do the restart and it says the volume is fine, so I'm hoping all is well.

edit: Trying to upgrade from 10.5.6 to 10.5.7 using the delta update killed my system. A kernel panic happened and it would never reboot. Not sure why this happened, but I suggest a clean install to get 10.5.7 working.