Going 64 bit and Vista you really need to get the memory voltages/timings down right. Not just the recommended ones, and what your motherboard is telling you (because the voltage can always be off slightly from the meter).

With 64 bit you'll probably be using more memory slots. More slots is a lot less stable, so you'll probably have to drop the memory speed from the recommended. The ratings given on the memory are usually for pairs of RAM sticks (I'm on DDR2, so that's 3 sticks for DDR3). I've yet to see any maufacturer sell memory in rated sets of 4 sticks (6 sticks).

Vista also makes use of all of the ram, all of the time, with it's cacheing of programs you might run, so it's more likely to blue screen than XP, which only uses the top end of memory when something big is loaded into it.

One of the other BIOS settings you should be looking at is Northbridge voltage (at least on Intel systems. AMDs, with the memory controller on the CPU probably have something similar), as the Northbridge is the chip that connects to your RAM. With a lot of memory sticks you get an effect called Vdroop, where the larger number of sticks pull more current, and as a result the voltage being supplied to them, and the voltage the northbridge is using to communicate with them, drops. Because of this you might have to increase the voltages for the northbridge and RAM a little bit, although you should never go over the RAM's maximum voltage. Most motherboards have a monitoring section in the BIOS, so you can see how close to your settings the actual voltages are (you may need to set BIOS settings to increase the voltages shown in the BIOS' monitor over what you need, as when the computer is under load Vdroop can cause this to drop again).


[edit] And yeah, BIOS updates are vital, as they improve how the system handles Vdroop among other things.