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I would suggest buying a smaller monitor for gaming. I'm using a 27in, but would say 30 would probably be the max if you plan on using this at a desk.
As for FPS, the 42in will not affect this because it is still 1920 x 1080 pixels just spread out more.
The Tv will affect the quality of the graphics though, for the pure fact that the pixels are so spread out, but if you are going to be playing from a distance, your eye will blend the imperfections for you.
Hope this helps.