There are cheap serial port cards as well. I guess couple of these would do the job:
http://www.usbgear.com/1x-Serial-PCI.html
The default trajectory planner and PID period in EMC is 0.001 seconds (1 kHz) so I guess it is sufficient update rate for motion control. 20 kHz is used only in motor torque regulation loop, not position loop. There is no point of running 20 kHz position loop when mechanical bandwidth of machine may be under 100 Hz.