Sounds like you want one PC to both run the machine and do your CAD CAM work? Pretty much any 1Ghz or better will run Mach3 fine, butCADCAM is another story. Most current processors will work just fine, the big difference makers are the video cards, which can be half the price of a PC for good ones.
I'd recommend a dual core processor, as they are pretty cheap and very common now, and can make a big difference when multitasking. 10 months ago when I bought a new PC, Athlon X2's delivered the most bang for the buck. All that changed when Intel released the CoreDuo and Core2Duo. Much faster and cooler running, and still relatively inexensive if you stay away from the fastest available. If you don't need the greatest and latest video card, a decent gaming card like a 6800GS or GT should be fine.