Well, first you need a budget. What, precisely, CNC? I'm aware of CAD. If that form of programming utilizes graphics heavily, expect to get a huge case and a high-end graphics card, combined, these two parts alone could cost as much as some low-end PC's. The HIS H697F2G2M Radeon HD 6970 2GB
is one of the most powerful cards on the market and should help. There are professional workstation graphics cards you could use, but those are far more expensive. If your CNC program enjoys physical cores, the AMD Phenom II X6 1090T Black Edition Thuban 3.2GHz
CPU paired with an ASUS M4A89TD PRO/USB3 AM3 AMD 890FX
motherboard would also increase speed. If the CNC program is 64-bit enhanced (Meaning, it uses more than 4GB of RAM) get as much DDR3-1333 RAM you can, which is four sticks. Depending on your workload, an SSD could also be considered.