As far as number of threads are concerned you are generally going to want 2 to 4. More than 4 seems to have diminishing returns and 2 threads is a lot better than 1. More threads also require a bit more memory to successfully complete a plot. The threading is only used in phase 1 currently.
As of Chia 1.0.4, RAM requirements are almost identical between bitfield and no bitfield. This is a chart of the various RAM choices assuming a k32 with 128 buckets and 2 to 4 threads:
RAM MiB: Minimum Medium Maximum
Bitfield 900 2640 3400
No Bitfield 900 2640 3400
Below minimum your plot will fail. Medium is enough RAM that you’ll get most speed improvements, but not all. This is useful when you’re trying to get more plotting processes parallel and have limited RAM. Using anything over the maximum is wasting RAM as you will not plot any faster.