All the b450 boards only support ryzen 1000 and 2000 from the get go. Sometimes the vendors update the bios of the boards. I would just be sure that the one you get has been updated before you purchase.
Numbers scary? Are you just meaning the pricing? If I remember, the 5700 is about the same performance as the 2060 super. So you are paying for the extra performance.
As far as the 5600xt, that card isn't out yet. But as far as the 1660 ti you had listed, here's a comparison to give an idea.
http://www.hwbench.com/vgas/radeon-rx-5700-vs-geforce-gtx-1660-ti
The 5700 is a better card. The wiser move might be go with the 3600 and the 1660ti, and use the money saved on a better power supply. But if you're going for a 1660ti, I would suggest have a look at the 1660 super which isn't far behind it and costs less.
What resolution are you gaming at and what refresh rate? If it's 1080p 144hz, or 1440p, I'd suggest the stronger card. But either way you go here, neither card is a slouch. I've got an rx 580 and it's still decent. Would like to upgrade, but it's not like I have to upgrade tomorrow.
As far as temps, I think gpus after normally good until 90-100 degrees. But that said, I don't know what case you have, but I would be sure that you have plenty of airflow around it. For example, I've got this case.
www.coolermaster.com
Not the rgb edition, but basically same layout. I have 3 fans for intake in front, 1 think in rear as intake. And 2 on top as exhaust. I'd have to check that. But mostly my stuff stays cool. But you can get things like pci slot cooling fans to put under the gpu, and use software to manually turn up fan speeds.