Rendering depend on ur cpu and ram.......normal/gaming gfx card is just for drawing the viewport.
20 min is not a lot but rendering time depends on the modelling ,materials, effects etc. I have had scenes which took around 3-4 hours to render on C2D @ 3GHz with 4gb ram and 64-bit OS and software (MAYA).
You must use professional gfx card such as quadro(nvidia) or firegl(ati). Those help in rendering but also cost a bomb compared to normal cards. Still I would say rendering depends a lot on raw cpu power.
Take a look at this......its a software based renderer for nvidia gpu.
http://www.nvidia.com/page/gz_learn.html