After adding more memory to my system (2 4GB modules), it begins crashing approximately every 12-24 hours with a 0x124 BSOD, which a quick Google search turns up to be a "hardware alarm". After doing some testing, I thought the new modules were bad so I sent in an RMA and got them replaced, but the exact same problem exists with the new modules.
I have 2 2GB modules, and 2 4GB modules (the new ones). There are 2 white slots, and two blue slots on the board. With some configurations it works with some others it doesn't.
2GB modules in white slots: Stable
4GB modules in white slots: Unstable
2GB modules in white slots, 4GB modules in blue slots: Unstable
4GB modules in white slots, 2GB modules in blue slots: No POST (I assume this is a "feature" of the board, the previous slots have to be full with identical or higher capacity RAM for later slots to work)
Modules in blue slots, white slots empty: No POST
All the modules are ADATA DDR3 1333 1.5v, the only difference is the amount of available memory. It works fine with the old modules, but any configuration with the new modules in it is unstable.
Motherboard: Gigabyte GA-P55-USB3 (default timing and voltage settings)
OS: Windows 7 Professional SP1 x64
Testing the modules with Memtest86+ overnight turns up no errors.