So I am building a new PC for RTX 5090. I was looking for CPU.
9950X is the same price as 9800X3D right now.
What's the point of 9800X3D if I will use my PC only in 4k+?
Isn't it better to go X and have 8 extra cores for the same price?
Not necessarily. The thing that many won't tell you is that the X3D CPUs are actually pretty weak. Their saving grace is the large cache which massively speeds up memory throughput, and a lot of the processor time is spent waiting on the RAM to respond. That's the magic behind these CPUs and why they are
sooo goood for gaming, 4K or not. But once you begin actually demanding chip resources, they generally fall flat because they're just 8 core CPUs with relatively mild clock speeds - this is where the Intel chips with their buff P-cores and plenty of E-cores + fast DDR5, or a 9950X with tightened 6000 MT/s (C26-C28) will shine.
Ultimately only
you know what's a priority for you, I target 4K 120 fps and my system should be more than capable of handling this with the 5090, while also being great to edit my clips and play around with my stuff without waiting an inordinate amount of time for the task at hand to be completed. If you mean to just play video games, then perhaps the 9800X3D would help reaching 4K 144 fps, or something. But I don't think even it would actually net that much of a gain.
If you're building a 5090 system, then clearly budget is no objection - and you should IMHO probably be looking at the 9950X and a tier S board like the ROG X670E Gene or X870E Apex. If you're going to strap a 5090 with a 8 core CPU, then perhaps just really do yourself a favor and buy the 5080, using the extra $1000+ you'll pocket to buy higher quality components, more storage, more memory, better cooling, better case, etc. and you'll probably have a better PC instead. That's my honest opinion as someone who actually did splurge on a '90.
To summarize:
Just gaming > 9800X3D (speedy cache chip to make games happy, AVX512 support will also help with some emulators)
Heavy gaming and mixed content creation > U9 285K/14900K (offers raw power)
Focused more on content creation > 9950X (lot of muscle and bringing in the big guns with AVX512 support across all 16 cores which will make a lot of renderers very happy)
You can't have your cake and eat it too, if AMD released a dual X3D CCD Ryzen 9, then you could, but they refuse, likely to protect their server business, so you can't.