Woah, Necro!
If it still has a 6850/70/* in there, that might warrant an upgrade, but what to is a good question. A GTX 960 is probably a safe middle of the road option, if you want to do that (might be overkill, but not too expensive, and has good enough compute performance to really help out, if it's not too much GPU for that's being created). But...
How much the GPU maters really depends on what is being done with the video, and the programs being used to edit and render. First, I'd check and see if the CPU is getting bogged down much, and if storage, which I don't see being precisely defined, in the past, is sufficiently fast. SSDs are cheaper than ever, and 4K raw footage might be a good excuse to look into HDD RAID 0, if the data won't comfortably fit on affordable SSDs.