Congratulations on the site and plans, nice work.
I already have a cnc machine, it is a combination mill and turning rig. Together with a friend we converted a conventional machine ( a picture of it can be found here: http://www.flickr.com/photos/jiskar/3969174253/)
We use linux with emc, works like a charm and it's free. But a paid solution like Mach3 can have it's advantages.
Well our machine is mainly for aluminum and steel, we have low-rpm spindles and relative low feed-speeds. Due to my interest towards parametric design and digital fabrication techniques, I want to build another machine, this time optimized for foam cutting and other light stuff. So your plans look very promising. Especially the simple methods and cheap method used to make the guidance system.
I do have a few questions.
What kind of slack do you encounter in the system?
How well do your x and y axis threading work?
What is you resolution?
What is your maximal free run speed (no cutting just moving the gantry around)?
sorry for all the questions.