moth_88

Pretty much what josh said for boards
Not always there are certain scenario's where you HAVE to use the cpu too

Yes it matters A LOT. The b550m s2h is gonna have thermal issues with the 3900xt. It's more a board that stops at the 5600x. What other boards are an option.
 
Ryzen does best with 3600mhz memory. Faster is pointless and lower performance the same way as getting slower memory does. So either get 3600 or something under it.

For mobo, maybe you can take a consideration on
 
MSI MAG B550M mortar 
MSI MAG B550 TOMAHAWK
MSI MAG B550 Gaming edge wifi.
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gigabyte aorus B550 elite
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Asus Tuf B550M
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Price range from 149 to 189.
 
All of them are decent motherboards with great features and power delivery at this price range.

fluid sim i use doesnt utilize gpu at all, and its like 70% of what i do, if not more
what mobo would you recommend? there are a lot i can pick from, but i hope to get the cheapest i can find that will perform well, hoping not to spend more than 300$ on that. maybe asus prime x570-p? its around 200$. theres msi x470 gaming plus max for 150$ aswell, but guessing its worth getting newer board.
i have no idea what to look for in motherboard but those are one of the cheapest x470 and x570 i can find. i consider getting some nice used last gen board over something from this gen but at full retail
 

The 3060ti + 3900x combo. Don't pay more for a 3900xt as it barely performs better than a 3900x unless it's like a 10$ difference.
 
Only lightroom would really benefit from the 5600x all the others would love a higher corecount cpu more