5, 5.5, 6, &6.1
That's w/ 64-bit Vista. Bryce doesn't benefit from 64-bit; but it doesn't hurt it either.
Bryce 6.0 and up are multi threaded. The multi threading doesn't work for the host computer when you use Lightning, Bryce will only recgnize one CPU on the host. The multi threading works fine outside of that.

EDIT - I guess I should say that is how Lightning behaves for me. It might be different for other people.

Friends don't let friends use booleans.