Needless to say, this took some tweaking. I'm on an MSI RS-480-M2 (SB400) on 10.4.6 (Myzar), and the first time I tried just dumping this file into /System/Library/Extensions the system would just reboot while updating the kextcache - wouldn't even get to the login screen. I had to use Macdrive (yes, it hates DAEMON TOOLS + SPTD) to get it to a usable state again. I wasn't even sure if it was this or the SB400 AC97 kext that made OSX angry, but i turned out I had skipped a lot while installing this. Do the following if you're on 10.4.6 (or higher?):
1. AppleGenericPCATA.kext and AppleOnboardPCATA.kext (the originals) are in /System/Library/Extensions/IOATAFamily.kext/Contents/PlugIns. Delete them (or rename so that they don't have a kext extension).
2. Dump the downloaded AppleOnboardPCATA.kext into /System/Library/Extensions. Do this from Finder, or in a root shell:
>mv AppleOnboardPCATA.kext /System/Library/Extensions/AppleOnboardPCATA.kext
The kext needs to be recognized as authentic (root:wheel, 0644; or so the manpage for kextcache said) so do this in a terminal as root:
>chown -R root AppleOnboardPCATA.kext
>chgrp -R wheel AppleOnboardPCATA.kext
>chmod -R 0644 AppleOnboardPCATA.kext
In case you can't get su to work, sudo each of these commands. If this step is skipped, kextcache skips the kext because it is "not authentic" (try kextcache -m to see what I mean). Next:
>kextcache -k /System/Library/Extensions/
>kextcache -m /System/Library/Extensions.mkext /System/Library/Extensions/
kextcache probably would have run on bootup, but I did it in the shell itself because I was too impatient. Now things work like a charm and it takes 15 seconds to load a usable desktop (Athlon64 3000+, 1G RAM) as opposed to 1min 20sec earlier without this kext. Splendid work, you guys. The folks who couldn't copy it in correctly, try the stuff I tried, this kext is worth a lot.
EDIT: I did not bootup natively immediately after this, I booted up in VMWare from the physical hard disk (just for fun). I'm mentioning this because when I tried the same things with the AC97 sound driver, the system would keep on rebooting during startup (native) when kextcache ran (why?). Again, in VMWare I got past that, and then when I booted natively I had sound and super-fast disk read/writes thanks to the ATA.
@Squimmy: Have you tried it yourself on 10.4.8, can you confirm that it is broken?
(sigh) now if only Darwin could use VESA 3.0 on my onboard ATi Xpress 200. I'm on 800x600x32@60Hz on my 7-year old CRT, and the flickering is going to drive me blind.