Anyone who played Halo CE will recognize this area.
This pushed my machine to the max. Max mem usage was 6.3 gigs. It took about 15 hrs to render.
Not 100% happy with a couple things, but Im not going to re-render after that long…
I would place it in the mountains 
example
http://www.conceptart.org/forums/attachment.php?attachmentid=751550&d=1250197608
Not bad at all; the range of trees up top looks a bit off in comparison (I know it’s a picture; but have you thought about doing some groups of “billboards”, using the existing trees in the foreground?), but overall a very solid piece.