What would be a worthwhile processor upgrade for this setup?

Going from dual core to quad core can make a difference but it really depends. For all intents and purposes, a quad core CPU has four separate physical processors, all running at the same clock speed; compared to a dual core CPU that only has two processors. You might think "sounds twice as fast", well sort of, but not completely. There are other factors at play here. As far as clock speeds go, the 2600k is a little bit faster, but not in any majorly noticeable way. The 2600k is able to handle more information at the same time, but that all depends on whether the software is programmed to take advantage of multiple cores and how will it can do so. The biggest difference between the two is the amount of Cache Memory. The 2600k has more than twice the amount of L3 Cache than the 3220 which can give you a pretty noticeable boost in performance in some situations. Please don't expect to see massive, earth shattering increases in speed with an upgrade like this, but you will notice a difference. All that being said, it really comes down to what you are using the computer for. If you are just using it for daily internet browsing and video streaming, then there is really no need to upgrade. Anything more than this and we have to consider some more specific details to balance your budget compared to the desired performance. Let me know in a comment what you are using the PC for and I will set you in the right direction.

It all depends on how you want to use your computer. The 1155 socket is quite old in the sense of how technology ages. Finding an unused CPU that is compatible will be kinda pricey. However, if you can find a new i7-2600k for a cheap price, you should be fine. It'll be on the same level as a mid-high end i5 CPU from the current generation. Good luck.