So there are a few ways you could go about upgrading this. I would start with getting rid of the 500 GB HDD and going for a decent SSD of similar or higher capacity. You will still be limited to SATA speeds since you can't use NVMe but it will be a huge boost over the 5400 RPM HDDs that are in there now. Keep the larger drive that you have for storage. You could ditch both drives for one larger hybrid drive that has an integrated SSD and faster HDD too, but personally I would only go that route if I were going to put 2 drives in there in RAID.
The next option you could look at is upgrading your 4x4 GB RAM to 4x8 GB, I know a lot of people say that 16 GB is plenty and most of the time it is, but having that extra overhead is not a bad thing if you want to do things like run Star Citizen and Discord at the same time. But this would definitely be after an SSD or hybrid drive upgrade.
Lastly on the cheapest end, depending on what cooler you have for the CPU you could overclock. If you don't know what you are doing with that read up on it or use the automatic settings that should be in your system BIOS. That CPU is unlocked and the MoBo has an OC utility you can use to make life a little easier. If you still have the stock cooler though,
do not OC the CPU. Get a better cooler like a Hyper 212 Evo from Cooler Master. You need to have adequate cooling before going down the OC path and that is a good air cooler to get started with. If you want to spend a little more money on cooling and your case supports it, get an AIO water cooler like a Corsair H60 and you can take your OC a little further. If you go this route, whatever cooling option you go with, make sure you use a really good thermal paste like Arctic Silver 5.
Ok, so that covers your cheapest options, what about if you can find some good deals or buy second hand? Look for a Ryzen 3 1200 and a motherboard that would support it. You will have better thermals, the TDP is like 30W less, the stock cooler can handle OC, support for DDR4 RAM, NVMe slots, and SLI/Crossfire support. In other words, you will have more expansion options down the road, but you are going to have to shell out more money up front.
EDIT: I forgot to add that if you are going to overclock, then in addition to good CPU cooling you need to make sure that you have good airflow through the case to make sure the VRM on the motherboard is cooled well too. That is something that can often be overlooked but it is just as important that it is cooled adequately too, because if you fry you VRM it won't matter how cool you can keep your CPU you'll be out a motherboard!
I also found this post on
Tom's Hardware that is answering a question specifically about the FX-4100 but is good advice overall for how to overclock a CPU. It can be time intensive to make sure things are stable, but can really bring older CPUs back to life so to speak.