As a general rule of thumb, a PC should only have ONE graphics card. Adding multiple cards can cause issues down the road and lead to further problems with power, etc. PC's that have multiple graphics cards are custom setups that require other preparations.
However, I work for a VFX studio and we utilize the NVIDIA Quadro K6000 units, and they do a great job by themselves with work even up to 4K. If all you're doing video wise is animation, they will be fine with just this one card.