I have the same barebones - SN68SG2 - with a 5600+, 4gb RAM, 512mb Radeon HD 3870. I am now running 10.5.8! I tried everything to no avail, but finally downloaded iatkos 10.5.7.
Install with Chameleon 2 bootloader
MCP67 driver under chipset
nVidia driver for ethernet
VoodooHD audio driver
Choose the correct video driver
The rest I left default
I was up and running in less than 30 minutes (skipped media check) and had all updates installed through Software Update within an hour. I am running all of the software that I purchased for the mac with no problems. Cannot run VMware, however, but I expected that. Make sure that you own a legit copy of Leopard and any software you have.
The only issues I have are:
an "unknown AHCI controller" in System Profile, which I understand does affect performance
had to turn off sleep mode
some unrecognized keys (home/end) on my Logitech wireless desktop (working on it now), but all volume keys work fine
Hope this helps