Of course your protein and computer will be different to mine, but just as a test I used a fully surfaced 380 residue protein with a docked ligand and did the raytrace you suggest:

using Pymol 1.2:
Macbook Pro 2.16GHz, 2GB RAM took 10 minutes to render, 20 seconds to write the 9000x6000 PNG file
RH Linux box (Quadcore Xeon) 4GB RAM took 5 min to render, 30 seconds to write the PNG

Didn't crash them, but boy did the Mac's fans spin! Could still do web stuff and read mail and compose this while it ran too, in contrast the Linux box was dead in the water until the render stopped.

-Martin

The requested size is 20" by 30". I calculated that it would correspond 
to a 6000 x 9000 pixel image.
How can I create such a large ray traced image without crashing the 
computer?

Dr. Martin Stoermer
Institute For Molecular Bioscience
Phone: +61-7-3346 2991