MacsRgr8
It's just that X-Plane 10, with extra highly detailed scenery add-ons installed, uses more VRAM than the 680MX can offer.
This really is true.
One screenshot is the scenery of New York City loaded, the other are my grfx settings including the amount of VRAM consumed (it is now 2587 MB, i.e 2.5 GB) once NYC is loaded.
You are doing something wrong. Reduce for example your "texture resolution" and you will get other results!
I reconstructed the situation you published (using B787) and got 778 meg !
http://www.flickr.com/photos/41672436@N07/11796788423/
http://www.flickr.com/photos/41672436@N07/11797303906/