First of all, thank you for your interest in Video AI
For Graphics cards, I would strongly recommend a recent GPU that is branded for gaming, think RTX 3060 or 3090 (or higher).
More robust hardware will improve overall performance and processing speed. Your computer’s ability to process a video may be affected by the size of the files and the number of frames. It is possible to exceed the resources of a minimally equipped computer and cause errant behavior.
If you post the original dimensions of the video you mentioned, e.g. 640 x 480 or whatever, we can give you an idea how long that should take you for upscale to say 1080p or 4K, for a given graphics card.
The suggestion of an RTX 3060 ti is a good one though, because they are currently heavily discounted and even an RTX 4080 or 4090 will only roughly double your upscaling speed (a bit less for the 4080, a bit more for the 4090). 3060 ti is the more cost-effective option, not the fastest as mentioned, but far far faster than your CPU alone.