Ray Tracing

Goddamn all of these homework threads. Let's talk about something interesting.

I'm starting work on a ray-tracer for shits, /proggles/ Any tips on speed improvements? It's being written in C.
I know the show stopper (usually) is having to check each element in the scene and see if it has been intersected by a casted ray. Any ideas on improvements in that area?

>better rendering algorithm with as much realism as a two-pass raytracer
You draw the objects in fast OpenGL code and apply "realism"(i assume shadows and lighting) globally like every 3D game on the planet.
Current cards all support shadows, transform and lighting, basically 99% of realism is done with a 10 years old card.
The rest of that 1% is the miniscule difference imperceptible to gamers and casual viewers.