Hmmm...except for the GI all the settings don't seem out of the ordinary. It's "better" to chose sample quality over GI detail, so in your case the scene would look even good, if not better, with GI @ 1/3. It would also render faster and use less memory.
Did you use any raytraced shadows in atmosphere or clouds?
Regarding the background-mountains. I don't use any photos for reference myself, but what I'd like to see is a bit more variation on colors. Normally I'd suggest to add some trees, but since your scene already crashes (perhaps not after changing your GI strategy) that doesn't seem like a good idea.
It would be nice to see some darker soil on the less steep slopes using intersect underlying in favor depression mode.
You could do it the other way around for the snow. Using favor rises for the snow will make it appear on the ridges as well, think that would look very good. Further you could add some stretched grass patches on the darker soil, by just using color (no models).
Just some suggestions to make it look as interesting as your foreground which is really good.
Martin
