How did you go with your two stepper Y with Winpc-nc.
I have a tangetal cutter using Mach3/smooth stepper. It has been running for two years now and I am not happy with the mach3 tangental control. Tangentail does not work on G0 moves and the look ahead for tangental moves is crap. My machine is T-Belts however I have one servo driving the X with a shaft connect both side of the gantry to the timing belts
So this past week I have been researching the Winpc-nc. It looks good for tangental. One thing I will have to get used to is drawing in dxf and converting to HPGL. I wish to build a large machine simlar to yours using rack and pinion and two steppers for the gantry. I here you about the extra cost for the Pro version with homing for dual steppers. The USB economy version looks really good with clock speeds to 80khz which i take it as the abilty to use faster rapids as the higher clock speed can process step/direction signals quicker.
It would be great to hear of your sucess with your project and to share your build experiences.