Are you playing on integrated graphics or do you have a separate graphics card?
RE CPU:
If you look at the boost clocks you'll see that the frequency is actually almost identical, plus the 7600 is based on the newer Kaviri architecture which has slightly better performance-per-clock. That means that the 7600 is slightly faster for lightly threaded tasks, and much faster when all cores can be used.
For Witcher 3 the quad core is going to help. Assuming your GPU is not the bottleneck right now, you will get better FPS a8 7600.
If you ARE playing on integrated graphics, then the a8 7600 has a much more potent GPU, but it's still integrated graphics that's going to struggle with anything vaguely demanding (Witcher 3 very much included).
Two other problems/things to consider:
1) What's the make and model of your Motherboard? The a4 6300 is an FM2 processor, while the 7600 is FM2+. If you have an FM2+ motherboard, you're fine, but my understanding is that newer FM2+ CPUs will not run in an older FM2 motherboard. So you need to check that your mobo is able to run the 7600
2) Depending on how much you have to spend to get the 7600, you're really throwing money into a dead-end platform. You may well be better off picking up second hand machine and throwing a new GPU in there.
If you pick carefully you can get an ex-lease desktop with a quad core i5 and just enough of a power supply for an entry level gaming GPU for around $100. There are RX460s available with rebates right now for the $70-75. Obviously you can get an a8 7600 for less - but you need to ensure mobo compatibility and there's no future upgrade path.