Mach3's velocity setting is what is used fore rapid (G0) moves, and is the fastest speed that mach3 will allow that axis to move.
Acceleration is very important, as it has a lot to do with how fast your machine works. If acceleration is too slow, you might never be able to reach the speeds you are attempting. If it's too slow, it can also have adverse affects in Constant Velocity mode.
It's up to you to determine how fast you want the machine to move.
Ideally, you want to accelerate as fast as the motors will allow. But you may find that too high acceleration may cause the machine to shake.