Remove everything but the video board, including all disk drives. Connect the keyboard and monitor, check the integrety of all internal connections and see if you get at least some beeps. Also, you may want to consider removing the MB from the case and powering it up on the bench, sometimes you'll short a conductor out in the case, it's happened to me on several systems. Basically, you start with a minimal configuration until you get it working, then add to it.