A graphic card simply has no influence on rendering time (beside maybe displaying the progression bar). Rendering is a numerical calculation that is done by the processor only. So what is important is processor speed and RAM. You never can have enough of that. And two processors are better than one, a second computer in a network is better than one etc etc. Graphic cards are important for scene setting and/or look at the results of the rendering. Thats all.

One day your ship comes in - but you're at the airport.