Thanks for the replies.
There is displacement for alot of the materials, should I remove/reduce it? The 32 bit memory addressing issue makes sense, though I thought the limit was 4.3GB, since 2^32=~4.3 billion, which in bytes is ~4.3 GB. But Maybe that was only for the OS, and it limits programs to 2GB? In either case I know more about hardware than software anyway.
The trees are pure geometry, not images or maps. Just edges and faces. That is one reason I thought I might be having problems, because they are geometrically complex. Sketchup doesn't appear to like complex geometry very much. I was able to render the scene using Twilight, but that had its own problems when the edge count later hit about 20 million.
Would it do any good (for rendering purposes) to trim out unused layers and materials? I know I probably should anyway, but could that possibly be causing any issues?
I want to point out that at no time am I using OS X. While I was using a Mac mostly for troubleshooting, Windows 7 x64 is the OS on both machines. Now if OS X might give me better results, I might have an excuse for a Mac Pro (or to convert my i7 rig into a Hackintosh).
I don't find that Sketchup is all that ideal for full 3D landscapes that are to be ray-traced and rendered, but it's great for a lot of other modeling, especially conceptual modeling. However, it's the only 3D modeling program I know how to use at the moment, and I have projects with deadlines.
Thanks in advance for any additional help!