You've already tried making the install drive and partition bootable?

http://osx86.wikidot.com/known-issues#toc3

It doesn't work? There are some alternate instructions / methods here and at insanelymac...

Booting from the Install DVD, pressing F8, and starting in "single user mode" ( -s ) to use "fdisk" has usually worked for me. Once you get to the prompt:

fdisk -e /dev/rdisk0 ( only 1 hard drive? should be "0"...)
update
f 1 ( if your partition isn't 1, replace with actual number - If you partitioned GUID it might be 2 )
w
q
reboot

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Running both 10.5.8 and 10.6.4 on my stinking AMD/ATi machine...