What exactly is a backplate?

Some coolers come with them, some don't. Yes the backplate will work with your case and motherboard. All it is really is a thin stainless steel "X" shaped piece with rubber on one side that attaches over the 4 mounting holes to provide a little stiffness to the mobo at the point where the weight of an aftermarket cooler could bend it.

Not always X shaped but for LGA775 they usually are.

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