New motherboard

I would do a bare post outside of the case on a phonebook to rule out metal to metal contact from one of the standoffs being out of position, or even the backplate metal tangs that sometimes have to be bent out of the way. I connect everything but the optical and hardrives. To post, you simply have to touch a flathead screwdriver to the power switch leads on the board. Another possibility is a memory stick that isn't seated or your board bios doesn't support your cpu. For beep codes, be sure to hook up the case speaker lead to the board. Many new cases don't come with a speaker, but you can get one at radio shack for a couple of bucks.