I have the same issue on both of my somewhat older (non intel) G5s. Troubleshot openGL and checked my RAM settings on both. No dice. I can’t even prerender 15s of footage. Oddly, everything works fine on my year-old laptop, but it’s ridiculous to switch over my workflow to such a slow machine. Until this is resolved, I’m switching my effects work to Apple’s Motion… an inferior program, but at least it renders!