What's a good cooler for this U of mine?

The FX series is a 32nm process that doesn't generate a lot of heat, and you have 4 cores. In fact, the dual heatpipe cooler can be fully compressed, but taking into account the stability and instantaneous cooling effect is recommended to use a 4 heatpipe cooler is sufficient. However, 4 heatpipe coolers are usually priced above 100, so it's good to find something cost-effective.

4 heatpipes of the recommended Nine Wind God Ice Ling 400, Cooler Supreme Storm S400 about 100 yuan

3 heatpipes of the recommended Simin CNPS5X Neptune Halberd about 70-80 yuan

The same number of heatpipes depends on the number of fins and the area of the heat dissipation, you can go to check it out.

Going online will be cheaper. Overclockers are not very cost effective and not recommended.

It's important to note that the silicone grease is applied, not a lot, but very evenly, and the fasteners are not too tight.

Good luck.