I'd like to share my experience with graphics cards.
I run Lenovo SmartCentre M710t, i5, 8Gb RAM, built-in Intel HD Graphics 630. No, it is not a machine for a video editing, but for my needs it does just fine. Before upgrading the video card, it exported a 6:52 min project for 18:58 min.
Now, I have GT1030, the maximum I can put in my computer, and it renders the same project for 9:02 min. Two times faster. The viewer runs much smoother (no stutter) even in Antialiased mode.
So, in my case I got a definite boost in performance. The interesting thing was when I checked a GPU monitor. The peak usage was only 32% and 15-25 in average. To be honest, I expected more. Not to mention 4 core CPU fooling around. Still, I feel much happier.