I am just setting up a Mach 3 system with a Galil motion Controller card.  So the hard work will be done inside the Galil.
My question is, "Is Mach 3 performance overly dependent on the PC speed and RAM size?  Is a multi-processor PC an advantage?".  The PC will be dedicated to only run the Mach 3/Galil combination, no internet connection nor other software.   

Dont think you will have an issue with any computer you can get today when using an external controller.
On two of my machines I use Via Pico motherboards with 1Gig integrated CPU and 1 Gig RAM, the Chiron uses a mini ITX mobo with a 1.6Gig Atom CPU and 1 Gig ram and the wee lathe and coil winder both use old point of sale all in one computer/monitor/touchscreens and they have 2Gig Celerons with 1 Gig RAM.

All have onboard graphics and all are networked to my desktop computer for easy file transfer.

Hood