no real guarantee when it comes to preventing 3rod.

just make sure there's some room for it to breathe.

I also wired up a couple of computer fans running off my computer's 12 supply, and that pumps in some cold air into the system. maybe that's why my xenon is still going even after a year and a half, touch wood.

to me, horizontal makes more sense, cos you're not blocking the intake vent at the bottom (right side, if you're facing the box). hot air rises, so when it's vertical, any heat not immediately evacuated by the fans/fan shroud heads up to the hdd area. it will eventually dissipate, but having one of the chips below the other chip may not appear to be the best idea.

also, have seen pics of vertically placed boards with the heatsinks removed, and with the thermal paste having slowly migrated downwards due to gravity.

either way, it's a fundamentally flawed design, so it's really a roll of the dice until MS brings out their more heat efficient designs.