If you are asking what is compatible, any modern graphics card is.
The issues come about when taking Power Supply quality and bottlenecking into consideration.
I had an a8 6600k a while ago, and I got a GTX 970, but I quickly got rid of the a8 6600k as it was not allowing the 970 to be used to it's full potential.
I would go for something like a Gtx 960 if your powersupply isn't absolute garbage, and if it is, you can either upgrade that aswell or get something like a GTX 750 ti.
An alternative, if you are planning to upgrade your CPU in the future, is to get the GTX 970, or radeon 390.