I like the way the question is phrased - minimum upgrade for maximum frame rate.
But I'm sure people are going to want to know what you current system is - CPU, video card, etc. Before they can offer constructive advice. Another important question is what resolution do you run at on your monitor? Are you 1920x1080 at 60Hz, or 2560x1440 at 144Hz, or some other combination?
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