replace graphics card for CS6

Here's the deal:

Your PC's 3rd-Gen Intel i5 CPU is stuck in no-man's land, as far as an appropriate GPU is concerned. You either overspend on a GPU that will continue to be supported in the future (think GTX 1650 or RTX 3050) but which may be severely overkill for the performance of your CPU, or settle for a GT 1030  which may be performance-matched to your CPU but completely lacks any hardware decoder or encoder whatsoever.

So, until you can afford a completely new PC with up-to-date hardware components capable of running the very newest version(s) of Premiere Pro, stick with John T Smith's suggestion of a GTX 1650 - and it doesn't matter at this point whether it is a GDDR5 or a GDDR6 version as both GPUs will be heavily bottlenecked by your weakling CPU.

perhaps I can get a little more guidance....two questions below...

my computer specs:  recently had to reinstall windows (now windows 10) and lost everything

Processor Intel(R) Core(TM) i5-3470 CPU @ 3.20GHz 3.20 GHz
Installed RAM 16.0 GB
Device ID 1EC55290-24A6-492B-9B63-A5F3A14D8375
Product ID 00330-80000-00000-AA902
System type 64-bit operating system, x64-based processor
Pen and touch No pen or touch input is available for this display

1st question:  since GTX660 is discontinued, what graphics card should I order for my CS6?

2nd question:  using intensity pro and blackmagic 9.6.8 did work for last few years but now not available.....which should I download, please.

thank you

Anything new will need to be added to the allowed list

If your nVidia card has at least 1Gig of video ram, use the nVidia Hack https://community.adobe.com/t5/Premiere-Pro/enable-CUDA/td-p/2649876 

You should buy one of the RTX series. Without knowing what CPU you have it is hard to make a recommendation.