Ciao chris1111 I want to share that I could eventually succeed in installing Yosemite on a very, very old mobo (see pic) with your guide as I already did on other builds. If anyone has the same old HP Desktop I would be pleased to give him feedback.
The bad side is that both on my gigabyte builds and on this one Preview doesn't work. It opens the files but if you edit them somehow you end with a messed useless thing.
In this latest build I have also some problems with mouse scrolling but it's not that bad. I'm sure I'll find a way to fix it.