The standard was AMD graphic cards for GPU mining - sorry Nvidia.
Now, Nvidia is producing faster video cards at great prices compared to AMD graphic cards.
To counter this, many mining software designers are make the Mining software for AMD graphic cards more advanced to help make up the speed difference.
So which is better Nvidia or AMD graphic cards?
I think currently the AMD graphic cards because of price and better mining software is better.
But if you are a gammer and plan to buy the best for both - then as of this year 2014.
I would get a Nvidia X9 series with crossfire support.
Then you have a great gamming rig - that can also mine while you are away from the computer.