That is more than likely your problem, it depends on your computer, mine can run no problem at 4KHz but Greg said 2KHz would be a safer bet.
For the Axis you want to set it to what each axis is, ie if you have in your motor tuning a step per unit of 20,000 and max velocity of 100 then that would be 2,000,000 pulses per min, divide by 60 to get per second and thats 33KHz so 64KHz would be needed as its the next above.
Hood