If you want to do your design in the same time you use your computer to run the CNC machine, this is not a good idea.
If you do your design and after that, use it to run Mach3, I thing they're no problem.
I think the dual core CPU will not makes much difference unless the software you'll use is programmed to use dual core.
To utilize a dual core processor, the operating system must be able to recognize multi-threading and the software must have simultaneous multi-threading technology (SMT) written into its code. SMT enables parallel multi-threading wherein the cores are served multi-threaded instructions in parallel. Without SMT the software will only recognize one core. Adobe® Photoshop® is an example of SMT-aware software. SMT is also used with multi-processor systems common to servers.
Jeff