I've contributed to a few threads on this topic in the past such as
this one and others - a quick search will reveal them.
...but don't listen to me, what would I know
Get the facts from none other than Dr. Don Goldman. His AIC2007 presentation is the richest source of information on the topic.
What You Always Wanted to Know About Narrowband Filters but Were Afraid to Ask - 1.6mb PDF - Enjoy!
FWIW, in speaking with Don some time back he suggested going 3nm for SII and OIII to provide maximum signal. SII can be a real pig to dig out of the background ADU and a narrow bandpass of OIII has no sign of impact at full moon. This of course requires longer sub exposures but the results speak for themselves.
Cheers