i54670k

I have a B85M-DS3H-A gigabyte motherboard and an i5 4670K, which I've overclocked in my bios to 4.3 GHz. In the BIOS, it correctly displays this clock, but when I boot into Windows 10 and check CPU-Z, it's at 4.2 GHz. How do I prevent this from happening?

I know my current build is pretty old now, but I don't have the cash to upgrade everything at the moment. I'm wondering if I can simply upgrade the graphics card for some improvement over what I have now, until I can replace the rest. I'm looking at getting either a Radeon RX 5600 XT or a GTX...