Re: X-51 BSOD - nvlddmkm.sys
I appear to be having the same issue. I had the NVidia website scan my PC, and it told me that I had the most updated drivers for the GTX 555, so I don't think updating drivers will resolve this.
Like the OP, this only occurs when I'm doing something low key like typing an Email, or browsing a news site. I can play Mass Effect 3 or Skyrim for 2 or 3 hours straight and it won't happen.