tl; dr;
Make sure your fans point the right way and you apply thermal paste properly.
When I bought my PC, I stuck am AMD 3900X 12/24 core processor in there and used the STOCK heat sink and fan. Out of the box, the processor ran anywhere from 40-60° C and I was pretty happy. Then I added a GPU and stick a few MORE fans into the case (at the top). When I was finished, the CPU was regularly overheating, spiking anywhere from 75-100° C.
This was annoying since my GPU was reading relatively cold 30-40° C.
I was ready to drop $200 for a water cooler, which I know would bring it down to 40° C (or less) even when in heavy use. But I obviously didn't want to spend the money on that since my upgrade list includes a second M.2 card and a second bank of 32GB of RAM.
I was talking to my son and he said that I'd put in t he top fans wrong. He said the best-practice is to draw air out from the top. I knew this was a best-practice too, but I decided against it because I wanted to see for myself and my intuition said that blowing cold outside air right onto the CPU heat sink was the way to go since those topside chassis fans are right next to the CPU heat sink.
Turns out I was probably wrong.
Another thing was to install those topside fans, I needed to re-set my CPU heat sink since it was "in the way". When I did this, I believe I didn't use enough thermal paste since I used the last bit of a syringe and had no more.
To make a long story short, I re-applied thermal paste and flipped the fans to blow out and now the PC idles around 45° C and heats to about 80° max under heavy load. That's still hot, but lowering peak temp by 20° C seems like a big deal for a minimal investment in thermal paste and a little elbow grease flipping those fans.
I wrote a simple Java program to saturate my CPU's. It just adds a counter so my gut tells me that lots of transistors in the CPU are unused and the program may not heat up the CPU even though the OS is reporting 100% utilization. It's possible a program that either access more memory or more registers/logic gates on the die could theoretically cause the CPU to be busier and thus heat up more. If anyone knows about this, I'd love to learn more.
I may still spring for a water-cooler down the road, but if this configuration remains stable then I guess I'm happy with it.