The buffer is the top one in the plugin, it is in Hz.
The axes are in KHz and should be set to the nearest to your actual axis frequency but has to be above and not below. In other words if you require 133KHz then you should set it to 256 KHz, setting it higher than required may not be an advantage and may actually have an adverse affect.
Regarding using the computer for multi tasking, it is not personally something I would do, computers are cheap and I prefer to dedicate them to the working of the machine only.
Hood