As long as you are happy playing in 1440p with some of your settings on ‘less than Ultra’ I think a 5700X3D would be just fine. I would go for the 7800 XT. IF POSSIBLE get a monitor that supports HDR. It really is a game changer.
DDR4 is fine. I used DDR4-3600/CL14 in my 5800X3D / 3090 Ti system running 4K with pretty high graphics settings. In case you don’t know, CL is an important number - the lower the better for a given RAM frequency.
There are several good motherboards out there for not much money. This video from one of the most respected PC dudes out there can help choose the right one.
One caveat… Don’t get a super cheap power supply. Look at this list and try to target something in Tier A if possible, but no lower than Tier B. Single rail is fine - and actually a bit better for gaming PC’s, as long as you get one with enough power overhead. A Gold P/S is fine. Higher levels like platinum and titanium are more efficient, but aren’t really necessary.
I highly recommend getting a Thermalright CPU air cooler. They’re quite inexpensive, but work amazingly well. The dual-fan units are pretty large, so make sure you check your case specs to make sure it will fit.
Speaking of cases, you don’t have to spend a lot of money, but it should have at least two intake fans (preferably 3) on the front, and one exhaust fan on the back. Good airflow is critical. I wouldn’t go smaller than mid-tower. I love my Corsair 5000D, but it’s a bit pricey. Here’s a video that can help guide you.
Have fun and Happy Flying! 