Here’s the install procedure I used to get Mac OS X 10.5.7 running on my P5W DH:

1. Bios settings: JMicron and Intel controllers in AHCI mode, Memory Remap enabled, Hyper path 3 disabled, CPU NX enabled
2. Boot from generic grub/boot-132 iso.
3. Hit Esc at boot prompt, swap disc for Mac OS X 10.5.4 DVD and press enter
4. install Mas OS, format partition if necessary.
5. After the install is complete, boot from boot-132 cd again. hit esc, type 80 then hit Enter.
6. should boot into leopard, at 1024*768 resolution.
7. Install bootloader: first PC_EFI 9 from bootloader.pkg, then Chameleon 2.0 RC2, with graphics detect option.
8. go to /Extra/Extensions, add AppleDecrypt, AppleVIAATA (for SiI3112 PCI SATA, Disabler (AppleUpstreamUserClient Version), remove A HCIPort Injector,
9. Install alc882 AppleHDA anrd ALCinject, pkg doesnt work, use kext helper.
10. update to 10.5.7 using comboupdate.
11. reinstall AppleHDA.kext