Noob with urgent job - a few questions

I need to render some caustics for a retouching job. I came across the Prism shader for cycles and loved the beautiful caustics it produces. So I downloaded Blender for OS X and jumped right in. Having never used Blender before I’m struggling, but I have a nasty deadline and need a result soon.

I’ve downloaded a dragon model, applied the Prism material, and put it on a plane. Then added a light and I’ve created some caustics! So far so good, but getting a decent render out is foxing me.

  1. How do I render the caustics alone, i.e. make the dragon invisible to camera?

  2. What will give me the fastest render - using my 12 core 3.46GHz CPU or using my 980ti with CUDA?

  3. How do I get a clean render of my caustics with no grain or fireflies - is that just the number of samples?

Thanks for any help