Yes, a 64 step motor will spin faster, but since each step is more distant, it is more likely to skip or loose steps. Since you can send step signals to the motor very quickly (computers are really fast compared to motors) I don't think there is any real advantage to using the 64 step motor over the 200 step motor... just step the 200 step motor faster. The trick is getting a motor and driver that can spin quick enough.

Mike is right that more voltage generally equals more speed, but calculating how much speed you can actually get depends on a lot of things, and more importantly, calculating how large a motor and drive system you need depends on knowing the total weight you are planning to move and how fast you want to move it. There is a lot of information and calculators to help you find all that at:

http://techref.massmind.org/techref/io/steppers.htm#Estimating

For example: CoAMarcus^ says:

"Pick the weight of the heaviest item you are pushing around. If it weighs 40lbs, use 40lbs. multiply it by the IPM [Inches Per Miniute] you want. Say that's 1,000 IPM. Divide the result by the magic number "531". The answer is 75.3 Watts".

Watts = IPM * Lbs / 531"

Volts = Watts / Amps, so once you know the power required to move the load at the desired speed, you can find a motor rated for an amperage that will bring the voltage into a reasonable range. e.g. less than 48 volts. And then you can find the minimum voltage from the motors rating and the wattage you need. (see the site above for more)

Finally, you can check to see how fast a specific motor will turn given the amperage, voltage and the motor type and inductance rating in bipolar vs unipolar modes using the calculator on that page. Bipolar gives more torque at low RPM, but unipolar generally will spin a bit faster for the same motor and power supply. See:

http://techref.massmind.org/techref/io/stepper/connections.htm for examples (bottom of the page) and discussion on motor wiring options.

Given all that, you can then select a driver that provides the amperage, and can manage the voltage you will need, and you can select a motor with the right inductance and wiring for the speed you want. It's a lot of information, but it's also good to learn all this stuff... Good luck!