Here's the good news: an only very marginally modified Mac OS X install from original 10.5.6 install DVD upgraded to 10.5.7 is working here, incl. hyperthreading, sound, ethernet, etc.
Dual booting with Chameleon 2.0RC1 from an HFS+ formatted EFI partition into Mac OS X and WinXP Professional also works.
Only significant change: installed an Apple WiFi card, because I want to use native AirPort drivers and network tools.
All that's needed is a specially prepared 132-boot CD, a bunch of extra kexts, of which all but 7 install in addition to Mac OS X' standard kexts, just like those for any other third party hardware.
5 need to overwrite Apple provieded kexts, but these are not required for booting, only to fix screen resolution issues. 2 must override Apple kexts, but if they are properly prepared, it's sufficient if these are in the Chameleon Extra/Extensions folder on the EFI partition.
The amount of extras needed to have a "as good as Apple" experience at this point is rather little, mostly some minor details around sound, power management and display drivers.
=> for just about $650 (incl. the purchase of a 500GB drive and Mac Box Set and new WiFi card) I have a machine capable of dual booting Mac OS X and WinXP and running 6+h on a single charge of the battery.