This looks good Bob, very nice!
It only needs some more atmo-samples. How many did you use now?
Regarding your render-problems:
I use XP64 and have 8GB RAM. When rendering my latest project I had to do 2 crop renders to manage the memory.
Each crop used 2.7GB RAM and 3GB VM. This is near the upper limits of the software but certainly beyond the limits of your configuration now.
So an upgrade would help for sure.
However, as you may noticed, rendering crops is an effective work-around as well. Depends on your patience as well

(my brand new day image was rendered in 6 passes)
I'd render it @ 1280x800 with 192 atmo samples and with 2 or 3 populations of grass (make them ~2-3 times smaller than the current grasses).
Split the render into 2 or 3 vertical crops and render each one after another with about 10% overlap (0->0.4 + 0,3->0,7 + 0,6->1 ...this always worked for me)
This will reduce the amount of memory needed while you still can use crazy settings or lots of populations.
Ideally, for example, you would also render this image with GI @ 2/4.
So there are still possibilities, it's just a matter of splitting the workload.
Cheers,
Martin