I recently upgraded a very old desktop system. It was an I7 860 16 gig of single rank udimmDDR3, with an rtx2060. All I did was a basic motherboard swap using the same Win 10 installation the upgraded to Win 11 afterwards. I bought a I5 13600k processor, an ASUS Prime B660 Plus D4 mobo, 64 gig of good ddr4, a case, and a few M.2 drives. Poser was not the primary reason I upgraded, but it was one of them.

Poser 13 really only needs an RTX gpu with enough memory to render the scenes you do. Optix rendering has come a long way and is faster than CUDA in P11. The rest of the system doesn't really matter if you render with Cuda or Optix, all the rendering is done on the gpu. I am getting away with a 2060 GPU with 6 gig most of the time, but not with every scene. Some have to be rendered on the cpu because they wont fit on the gpu. I plan on getting a 4060ti with 16 gig whenever it comes out. It will be a very inexpensive card for what it is, the amount of memory it has, and the power savings over the 2060. 

No one can really give a definitive answer to what is best for you without knowing more of what you do on your computer outside of Poser. If your computer does everything you need it to now, do you even need a new one? I play games on mine and some of the newer titles just would not work on a 1st gen I7, some software I use a lot went the same route, (no avx extensions in the ancient I7 860 cpu, programs wont run if that is required) so I upgraded the mother board to a fairly inexpensive 13th gen system. 

If you prefer to render in Firefly and don't need the gpu to render, do you even need a faster processor or more cpu cores? You didn't mention what cpu/system you have now.

As far as why you have to upgrade from the previous version. That is up to the company that owns the software to decide, and I am not really sure how that works with Poser. I have every version they made except for the first that was mac only, so I never ran into that.....

I didn't upgrade my Poser system for nearly 15 years, other than upgrading gpu's. Maybe that is all you need to do, maybe not. A 2000 series RTX is a bottle neck on an old I7, and it showed in gaming. A 4000 series would be a huge bottleneck on any system with PciE 2...... So I upgraded, basically because technology and program's left my ancient I7 in the dust....

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