If you intend to capture from your own PC and not from an external device like a console, a capture card does not give any advantage. In contrast, it's worse than the internal capture methods of OBS (game capture, window capture, display capture). A capture card needs more resources and produces more CPU load than these internal capture methods.
If you want kind of "invisible" capturing of a PC game, you need a 2 PC setup. One PC with your game and one PC used for streaming. You put the capture card into the streaming PC and capture the video signal from the game PC. Keep in mind that it's not possible with every capture card to handle an input signal with fps higher than 60. If you have one of the new 144 or 240 Hz monitors, a capture card may choke on that signal or prevent the computer output a signal with more than 60 fps. No such problem exists with the internal OBS capture methods, although the video may appear to have a slight stutter or judder if the game fps isn't a multiple of the video fps.