Hi,
the video card doesn't have anything to do with the CNC part but it relieves the CPU of graphics tasks and therefore
the CPU can do a better job running the CNC.
If you disabled the toolpath display that would minimise the CPU load and for very large files with demanding redraws
it is recommended otherwise the CPU can run out of resources and the CNC will stop/stall.
I ran a single board PC based on a dual core Atom processor, very low computing power and even worse relied on
'on chip graphics', the worst of both worlds, with Mach3 and two parallel ports for a couple of years without major
problems. Just don't hit <regen toolpath> while machining. Just goes to show that even a very modest PC can do a good job.
I now use an ESS and things are better still, the motion is smoother, the motors run quieter, sweeter and cooler,
never have a problem with redraws.
Craig