I'll answer my own question here :-)
My problem was with the southbridge heatsink being tightened too much. I guess either a connection was being broken, or two points were shorting. Either way, loosening the heatsink screws a tiny amount "fixed" the issue with my new board, and it boots fine now with both CPUs and a maxed out memory configuration. I've been running Windows 7 RC on it for a couple of days now, and no problem.