I followed the instruction on post #2
My configuration has a Gforce 750Ti, so after install the update and reboot i got the screen frozen while charging the progress bar.
I had to get back to the onboard graphics of my GA-Z97 with "GraphicsEnabler=No -x maxmem=4096" to get login and complete the yosemite update. Other ways drive me to kernel panic.
After that, put again the gforce750Ti and boot with only "-x" to be able to login again into yosemite. Install the beta drivers for Nvidia 10.10.4 Yosemite and reboot.
All ok after that, one hour of wandering but i need it to use xcode beta!!! Thanks to all!!!