Apple, fresh from the G5 mobo peripherals bottleneck fiasco, went with an Intel motherboard.
There
is an article at Extremetech.com:
http://www.extremetech.com/article2/...1886868,00.asp
... that describes building a Mac -- but around a 3.8 GHz P4, apparently similar to the machine Apple was using to demo OS X for Intel before the Core Duos hit the runway.
But that's pretty far afield from what you want to do. Kind of an interesting read, though.