Okay, finally some good news. The render is now past the point where it had failed before. I at least have the top of that mountain on the left there and still going strong. Also, I have left the bucket sizes and other settings at their defaults too (instead of lowering them), so this definately seems to be a memory issue. Filling in that last slot of RAM'll probably do me. I didn't know how to set up to record the way memory was being used, but I have kept an eye on it. At the start of the render I was at 800,000 to 900,000. When the render was approaching the point where it had failed before, it dropped down to 300,000. At this point, I thought there might be a memory leak in the program, but now, as we can see, it has jumped back up to 600,000 or so.