This is a very interesting topic, analyzed here:
http://www.hardballtimes.com/the-relative-importance-of-velocity/
If you don't want to read the whole thing, here a summary...
Fastballs - location is 2x-3x more important than velocity, but velocity is 1.5x-2x more important than movement. However, you can't teach velocity, just like you can't teach raw power or make a basketball player taller.
Curves - importance ranks velo, vertical movement, location, then horizontal movement.
Sliders - importance ranks location (down), horizontal movement (away) and velo last.
Change - keeping it down is by far the most
Important aspect covered in this analysis, but common sense tells us the velo delta must approach 8-10 MPH, and the delivery must match the delivery of the FA.