Huh.. you doing it wrong. Maya is not designed to work with such a high number of polygons in the viewport.
What you would rather do is export a displacement map in mudbox, and your low res model (or maybe sdvl 1 or so) to maya and set up that displacement map. So that the model gets subdivided and highres at rendertime.
Or you can do that with a normal map, thats the easier and faster way to render because the detail is just a fake of light and shadows.
For your other question: A good graphics card will increase your performance in the viewport,
when rendering (i.e. with mental ray) its the processor and ram that will influence the rendertime.
So everything comes into play for general workflow speed.
Be Happy 🙂