"The best Nvidia card you can justify". OBS needs the GPU to render frames even if you're going to use the CPU do compression. If you just stick with built-in video or a cheap or old discrete card, you may find that your GPU cannot render frames fast enough to feed the compressor at the frame size, frame rate, and quality your CPU can handle compressing.
Making that an Nvidia card gives you the option to use NVENC if you choose. There's no particular advantage on Windows (unlike Mac or a Hackintosh) of using an AMD card, but if that's your jam, go for it-- any decent card is better than none at all.