You might be able to get by with the power supply but 750+ watts is recommended for a 3090. Any B550 motherboard should be totally fine.
For what it’s worth I’m running a 5950X, 3090, a pair of water pumps, and 11 fans all off a 750 watt Corsair PSU and I’ve never experienced any instability or signs of an insufficient PSU.
If it were me in your situation, I’d just pick up a decent 750-800 watt PSU, and upgrade the memory to a decent 32 gig memory kit while I’m at it just to have a little better time. 3600mhz with decent timings seems to be the sweet spot. Otherwise, as long as the GPU fits the case, you should be in great shape to run it.