i7 930 x21 multiplier question

I was wondering if anyone could tell me whether there was any Pros/Cons to working with either the x20 vs x21 multiplier on the i7 930? I have seen about equal postings where some have left Turbo mode enabled and others say most definately turn it off. If off, does the x21 multiplier remain available? I also wonder about Speed Step...on?...off? I want to get somewhere in the 3.8-4.0ghz range using the lowest voltage possible. Right now it's just a general question as this is a new rig (listed in sig) but I'm still waiting on a few of the components. LOL at least during the wait I've had enough time to pour over the manuals and forums. Any explanation on the above would be much appreciated.

Some say the odd multipliers are better but......its what works for you.