the two most important parts of the x-step are to:
1) use your hips
2) get your weight forward.
how much run up people need depends a lot on physical abilities.
the more athletic you are, the less motion you need to get up to speed. e.g. mark madsen on the timberwolves is one of the fastest guys from endline to endline... but it takes him 4-5 steps to reach speed. allen iverson on the otherhand probably isn't as fast in raw speed, but can accelerate to 100% within 1-1.5 steps.
that being said, timing becomes the #1 factor in "hitting" a throw.
while there lies more potential at greater speeds, the window for "good" timing becomes increasingly small. it is possible to throw over 430' using simply finish power, but to go 700' you will need whip + finish power. however, it is not possible to go 430' simply on whip power.
if you watch enough DG dvd's you'll find that all big throwing pros have a great amount of finish power, regardless of the amount of whip they have. for example, markus kallstrom has an insane amount of finish power, but he doesn't have nearly the amount of whip that shawn sinclair has. one thing they do have in common is a very strong finish.
to sum it up, you NEED a strong finish. the faster you can go WITH a strong finish, the better, but as you go faster, a well-timed finish will become increasingly difficult to feel.