changing premier pro video card will work

Nothing is going to save that 11-year-old CPU platform. That so-called "8-core" CPU actually performs worse than even a 2nd-Gen Intel i5 CPU-based PC - and both are woefully inadequate for video editing of any kind these days. As in, that FX-8120 CPU will severely hold back the performance of even a budget GPU that's even three years old! Even worse, it performs worse in video editing than even today's cheap laptop while consuming umpteen times more electricity!

A good example of what I am trying to say is the few PugetBench for Premiere Pro results on that FX-8120 that are in the PugetBench database: The 8120 scored barely above 200 (compared to the over 1,000 score that a good modern current-gen CPU-powered PC regularly achieves) in that benchmark test using the "Standard" preset (and that's with a GeForce GTX 1070, which is more powerful for video editing than the RX 580 due to Adobe's lackadaisical support for OpenCL while favoring Nvidia-exclusive CUDA). That puts it roughly in the same performance class as its contemporaneous Intel i5-2400 counterpart which has only half the cores, half the threads and a slower all-core sustainable clock speed as the FX-8120.

In other words, it's time to retire that entire PC ASAP. Do not waste a single penny on that relic.