The CPU / Processor is the main brain in your PC. It's doing all the number crunching. Assuming there are no obvious weak links in the chain, upgrading to a faster CPU / Processor will make the biggest increase in performance for your computer.

There are generally two brands that dominate the CPU / Processor scene: Amd and Intel. In my experience, both are 100% capable of delivering tremendous audio performance. One company tends to be quite a bit more inexpensive than the other. I'll let you figure out which CPU processor you prefer.

Processors have a “cache”. Caches run in all kinds of sizes, but generally speaking, the bigger, the better. Research this more. I'm not an expert on CPUs by any means, and only you can decide which CPU is best for you.

Just make sure that you select a processor that works with your motherboard.