since your goal is 10.5.7 i would recommend to get iatkos v7 which is 10.5.7 already.

then as a first step after booting the dvd fire up disk utility and to keep it simple make one partition, format as hfs with journaling and choose master boot record as partition type.
click apply and quit disk utility. now the installer should be back and you're good to go.

choose chameleon, dsmos, smbiosenabler, a disabler and whatelse matches your hardware. also be sure to install one of the patched kernels because vanilla won't work for you.

you might wanna get the detailed specs of you board first!

after that you might wanna come back here and tell us how it went so far.

good luck