For an RTX3090, 850W minimum I would say. RMx would be a solid choice again.
This would be a very unbalanced build though, you would only want an RTX3090 if you are running 4K resolutions or have a need for 24GB of VRAM. The upcoming RTX3080Ti is more suitable for the casual gamer, but even then, If you are sticking with 1080p, than an RTX 3070 is about where I would draw the line, and 3060 Ti, and 3060 will do just fine as well. 3070/3080 are more suited for 1440p high refresh rate gaming.
B550 board will let you upgrade to a Ryzen 5000 series processor later, which would better match up with something like the 3090...