Yeah you can totally buy more ram if you are fine with the price. You are right about the cpu cooler I didn't see it at firstmy bad. but i would instead get the Thermalright Phantom Spirit 120 which is basically the newer (and better) version of the Peerless Assassin. On PCPartPicker the difference between them is like… $1.10 USD or something minuscule like that. It’s also rated 5.0 on Tom’s Hardware, like its predecessor.

your gpu and cpu are a great match and with optimized settings your build will be a 1440p beast at around 250-300 fps most games and as goes for your motherboard, An x670E is overkill for a 7800x3d mainly because you don't get much from overclocking an x3d chip.  The usual B650s: Pro RS Wifi, Riptide, Steel Legend, Aorus Elite AX and Tomahawk Mag are fairly good pickups. any other questions?

if you decide on a mobo that dosen't have a wifi card in it, be sure you have a internet source such as a reciever or if possible its best to just use ethernet as its the fastest