Guys,
I suggest the "KFLOP" from Dynomotion for a USB controller. This controller is built around a very powerful DSP. Has it's own CNC(Free) front end but there is a Mach 3 plugin available as well. Recently the manufacturer added 8-axis control capability. Dunno why it's not mentioned more here on the 'Zone.
see here;
http://dynomotion.com/