If you need to speed up things, you need to increase memory to 32 GB.Adding another GPU is a waste of money.
The CPU is one of an older generation and adding one is also very costly and requires upping memory to 64 GB, while the performance gain is limited.
Thanks for the info and the links. They are very helpful! I am definitely going to max out my ram.
Reading them, though, it looks like a video card would be helpful in my situation. I do a lot of frame rate changes (mostly converting non-24 fps camera footage to 24 fps...I do this on just about every project I work on) and scaling (2k to 1080, or even 1080 to 720). Also, while export time is most important, I would also like to stop the jittery playback while editing, and it seems the video card would help that, according to the link (I use virtually no third party plug-ins).
As to the CPU, 3 years ago it would have been expensive (since the 6 core Xeons ran around 1000 bucks a piece), but not today. Adding the additional CPU will end up costing me much less than 200 bucks. Far cheaper than any other upgrade I can do.
While the 570 is not slow at all, you may consider replacing it (not adding it) for a GTX 970 if you have added the 2-nd CPU and increased memory to 64 GB.
Adding a 970 to the 570 means that half the acceleration is done by the 570 and the other half by the 970, effectively lowering performance over a single 970, because the dual CPU's are too slow to feed the 970 to full capacity.