Let's say you have such a build:

1x Ryzen 9 3900X
2x G.Skill Trident Z Neo 16GB
1x water cooler with fan
1x SSD 1TB, 2,5"
1x ROG Strix B-450-i mini-ITX motherboard
1x RTX 3080 Founders Edition

We recommend at least 743W of power for this setup. If you buy a 750W power supply, you will feel the heat under full load. A power supply between 850W and 1200W is recommended.

Most 3080 cards will not fit, but the TUF from Asus, the XC3 from EVGA and Ventus from MSI will. Here is a table for all 3000 cards: https://www.reddit.com/r...

As for the power supply, 650w should theoretically be enough, but you won't be able to add any major upgrades to the system in the future (mine with Ryzen 7 3700x uses about 480w with Rtx3080 TUF).

I hope this helps you.