i have managed to figure out one thing i did wrong: i screwed the motherboard directly to the motherboard base plate.
the coolermaster cavalier is a pretty nice case (though the little metal pieces you have to break off to add drives/cards are sort of flimsy), but the documentation that came with it was beyond awful. for screwing in the motherboard, all it said was "screw in the motherboard using appropriate screws"... i searched & searched & only found four screws thick enough to go into the holes in the case. sure, there were also these weird brass things; they were to screws what platforms are to shoes. but i had no idea what they were or what they were for, and the instructions didn't say, so i didn't use them.
those "motherboard studs" are used to seat the motherboard a few centimeters away from the case itself... i imagine that if the board touches the metal case in the wrong spot, that can reroute the current or even cause a short circuit. logically, this could be the cause of all my problems. (the power supply isn't as strong as i'd like it to be, but 350w should at least be enough for it to boot up!)
i guess tomorrow i get to unscrew and reseat my motherboard, then cross my fingers that it boots...