OK, have some updates, Xeon works just fine in these motherboards, highly recommended. My problem was (after a few hours of try an error) - stupid reset switch in the case, while connected to the MB, was not letting it boot! Found it by disconnecting everything by coincidence!
I'm having a similiar problem as you. I've build a pc with a ga-p55m-ud4 motherboard and an iCore i5 750. When I press the power button of the case, the board starts for 0.5 - 1 seconds and shuts down. After 2 seconds the process begins again. No post no bios... I've tried to disconnect the 'reset SW' cable on the board but it won't help. Do you have any advice? Thanks.