Will socket 771 Xeons work in 775 Xeon motherboards or regular 775 motherboards? Also, on another note, will 1U Xeons work in dual socket 771 boards? Third, will 775 Xeons work in P35's and such?
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No to 771's in socket 775.
No they will not, probably there will be no POST, less likely the Xeon or the mobo will get fried. About the second Q, I honeslty don't know. About the third Q, perhaps the ones rated at 1333 and 1066 Mhz will, but i'm not sure.
I would check with the Mobo Maker for the 775 question in regards to the P35 question. It may vary from board to board.