Not sure if this may have been answered yet, or if anyone will agree, but Vista will nearly fill all of the physical RAM up and then start on the pagefile. That is probably why Vista will BSOD with those sticks and XP wont, xp usualy wont load everything it can into the physical RAM. So you can probably see why Vista is crashing more with the 3 or 4 sticks than what XP would be.
EDIT: BTW, try running Memtest 86+ or vistas Memtest program (they are actualy the same program I think). Let it run for a good 24hours in a normal enviroment. This way you can really make sure it IS the ram... Better than trying to flash a bios and possibly ****ing up.