For noobs who might be following this thread, we now know that you do not need the DSDT patch in order to install the 10.5.6 update, that was just for the betas.
You can install the Retail Leopard DVD by booting it up with a grub-dfe boot-132 disc. Install with the boot-132 disc, then place that mkext file into the /Extra/ directory. Also make sure u apply the DSDT patch (~pcwiz made a very easy GUI tool for that 2-3 days ago) so u can update.
RJ, thank you for all the info on the Gigabyte GA-EP45C-UD3R board. Just replaced an ASUS P5W and following these threads made it easy! Doing the Boot-132 option now.