If your system doesnt work with the modded AppleIntelPIIXATA I would say your problem is either:

- It has a device id not present in the info.plist (doubt it, its supposed to have all ICHx ids)
- You have some other ATA kext installed that is causing a conflict, taking over it.
- You have some mismatched kexts versions between /Extra and /S/L/E, and the wrong one is loading.

The last one is a possibility (I use EFI, not Extra, but I see what you are driving at). I think what I am going to do is :

1. Install 10.5.6 the way I am doing so far.
2. List everything in EFI and everything in S/L/E and see if there are any matches.

The reason is that this thing goes south only after I upgrade to 10.5.6. 10.5.5 works fine.

It has to be an EFI kext, because my understanding is that they are loaded first, and any matching S/L/E kexts are ignored (or do they over-ride ? - suggested by the exhortation to delete the CPU Powermanagement kext from S/L/E in the netkas instructions).

Device ids. ? How do I check that ?

If it only breaks after upgrading it probably means the update installed a new unpatched kext in /S/L/E and that one is overriding the one from /Extra.
From what I understand kexts from EFI/Extra are loaded first but the same-name kexts from /S/L/E are only ignored if version is same or older, if they are newer they are loaded and override the ones from /Extra.

Dell Optiplex 745 and iDeneb 10.5.6
 

I finally managed to get Optiplex 745 to work with iDeneb 10.5.6 and even 10.5.7.

Here is how I did it.

Steps taken for installing iDeneb 10.5.7 on Dell Optiplex 745, Core2Duo 2.66 GHz, Broadcom NetXtreme (MB) and RTL8139 network card (PCI), Soundmax HD sound, Intel Q965/Q963 Graphics, USB keyboard and mouse:

0. Step 0 for me is to have OS X in second partition, first partition is Windows XP. I use MacDrive so I can see everything in the OS X
partition from Windows. I use chain0 for dual boot, Windows XP or OS X.
1. Started with clean iDeneb 10.5.5 install. For patches selected minimal set, tried for only what is needed: I selected the intel chipset, IntelGMA graphics, for sound I selected AC97 and azalia (I didn't know which one), for fixes I included all (note cpus=1 is selected)
2. Install and bootup was flawless, iDeneb 10.5.5 is fantastic for that! Note for my monitor I had to specify "Graphics Mode"="1024x768" at bootup, then I had to modify boot.plist for that. Internet worked (RTL8139) and sound worked immediately.
3. Then I prepared for the 10.5.6 installation, by using PCWiz OSX86 universal installer, and DSDT gui patcher. Then, I updated with iDeneb 10.5.6 upgrade kit.
4. Things get chappy now. I had to use Chameleon 1.0.12 boot loader and voodoo kernel to get it to boot. I did get the PCGenUSBEHCI errors, but the boot went through twice, the first time it rebooted at the end. Note the flags "cpus=1 -f -v" seem to be
necessary for boot.
5. Once I rebooted a few times, things were OK, I continue to need to set the verbose mode, -v, and -f. Somehow it needs these!
6. Then I upgraded with iDeneb 10.5.7. Very similar to 10.5.6, requires the double boot at first time, and same flags, cpus=1 -v -f.
7. After this I updated my installation through Apple. I selected what I needed.

Note, for me the first step of installing cleanly iDeneb 10.5.5 was a must. I tried this with a sloppy install and things did not work well!

If someone can comment or enhance my process I would appreciate the feedback. I am particularly interested in the flags (why -v -f cpus=1) and fixing the PCGenUSBEHCI error.

Regards....

Thanks for the info. However, I do not see PCGenUSBEHCI errors. Plus, this is a dual core system. Can't really use cpus=1.

Currently do have the time to do extensive experimentation, but none of the things you mentioned have anything to do with the chipset. Plus, I am using retail DVD.