GTX 660 would be more than enough, but if you want the most modern card, even though it is somewhat overkill, look at the GTX 960. An extra, large capacity, SSD would be nice to add.
I would seriously consider a GPU with at least 4 GB of video RAM for your 4K media. Also CC does much better with 4K media tha CS6.
Thanks for the suggestion. Are there any differences I should consider in the 9 series, 960 vs 970 vs 980?
960 is slightly overkill. Sometimes idle.
970 is serious overkill. Regularly idle.
980 is way overdone. Most of the time idle.
The more you spend, the more unbalanced the system becomes. Your system can not feed the video card fast enough to use its potential.
The 970 GTX would not be overkill with the 4K media atleast not that much as long as the CPU is clocked to 4Ghz.