you can use gpu-z to see how much the radeon hd 6970 is currently being used in premiere. premiere uses the cpu to decode the footage/codec, so chances are the cpu is your bottleneck. you can check with windows task manager and look at the cpu usage. transcoding to dnxhr or making dnxhd proxies might allow you to continue using your existing computer, if you don't want to upgrade right now.
if you want to upgrade the video card, the gtx 960 4gb or gtx 970 are good options. quadro advanced features aren't needed.
a new 4 core i7, like the i7-4970k or i7-6700k, will be around 50% faster. the x99 platform has more cpu options for cpu's multiple times faster than your current i5. the i7-5820k is a good x99 starter cpu for 4k projects/media, and is only a little more than the 4 core i7's. it has 6 cores and is around 2x faster than your i5. there are tons of x99 build threads on here to look at too.
Thanks for that. I did what you said for playing videos on the source monitor, program monitor, and in VLC. Clips coming out of Mercalli are choppy on all three playback methods with 50% load on cpu and memory. GPU load is <10%. Everything is happening on one HDD (Seagate Barracuda 7200)... Is this why? Video straight from the camera is only choppy in the program monitor with same loading percentages. Also, memory usage is just under 8gb before playing video and just over when playing video; does this indicate anything?
Can I edit my post in this form? Don't see any button.
But wanted to add that the choppiness goes away in the program monitor (raw and processed footage) after I preview render it.
you should be able to edit a post by clicking on the actions button at the bottom left.
are you using the mercalli stabilization program? if so, what format/codec are you exporting to?
if its already using 8gb of memory before playback, premiere won't be happy with so little being available and can impact playback.
the seagate hdd might be ok, it will depend on the bitrate of the file. you can check read and writes activity in windows resource monitor while playing back the video files. you can compare that against the hdd's max read and write with a program like crystaldiskmark.
just about every part of your system would be considered a weak-point for 4k editing, and would be considered barely passable for HD editing. you could upgrade a few parts, but it would be better to think about a new system. transcoding out of h264 to dnxhr or cineform, or lower to HD proxies with dnxhd may be the only way to get your current system to function smoothly. if you haven't already, you can also change the playback resolution down to 1/2 or 1/4. the preview render is creating a new file, think of it as a proxy file. it just shows that another format/codec will function better than the source's h264.