Why doesn't blender use my graphics card? Before installing a graphics card, I was working on a very weak intel card. I tried to render a scene but the quality of the image was poor and it took 1 minute to render.

After buying and installing Nvidia GT 710, I tried to render the same scene and I was expecting to get better results but I got the same results. 

It seems that blender doesn't use my new graphics card. How do I know which card does blender use? How to switch between them?  

Edit:

I like the answer but I only have the CPU option. Version: 2.73a linux 64-bit.
In Blender head to File -> User Preferences -> System.

At the bottom left under "Cycles Computer Device" choose "CUDA" and you should see your GPU underneath. You need to be using the Cycles render engine for this to take effect.