I have been doing more video editing lately, and as I learn about the different video effects and compositing, I am using those techniques more and more.

I do use proxy clips when editing.

I recently bought a new video card (but haven't installed it yet). Until now, I was using the onboard video on my motherboard. Then a friend told me that video editing software in general uses the CPU and that a new graphics card will not help much. I did some searches about Kdenlive and graphics cards, and they seem to agree with my friend, though I could not find anything more recent than 4 years ago.

I would like to improve Kdenlive performance in the following ways:

1) No more choppy playback when I am previewing my timeline (more important)
2) Faster rendering times (less important than #1)

Current specs:
CPU: Intel I5-2300 2.8 GHz quad core processor
RAM: 8 GB
GPU: Motherboard's integrated graphics (don't know specs on this one).

If the new video card won't help, will upgrading the CPU or RAM be more helpful?

I actually ordered a new computer yesterday, hoping it will improve playback in Kdenlive as well.

More RAM capacity will not speed up playback unless you're constantly running out of it (not likely with 8GB when editing 1080p and not having 20 flash video tabs open in Chrome - as a test you could shut down, remove one of the RAM bars and check performance with less RAM). Not sure about upgrading RAM frequency or delay (class), I'm assuming no difference.

Upgrading CPU should increase playback speed.

Upgrading graphics card should upgrade playback speed if the previous graphics card was so weak it would slow down 2D rendering (that was the case with my onboard AMD graphics card and two 1080p screens although I might be misidentifying the cause).

Also there is a GPU rendering method, which you can enable in settings/playback (video with instructions: https://www.youtube.com/watch?v=47maykCIo5w )

You will have to have "movit" installed. Unfortunately it is currently very unstable in my experience, Kdenlive often crashes in my case and some other users reported the same on the bug tracker and in comments of some videos I made about the Movit GPU Kdenlive functionality:
https://www.youtube.com/watch?v=cip3MOjSfCs (GTX 780 proprietary seems to having been crashy in the past)
https://www.youtube.com/watch?v=laSHZ0hVVJY ( Nvidia GTX760 proprietary seems to be ok now)

At this time, MLT is a real bottleneck despite asking it to use multiple threads. MLT can only paralize a few things, such as the frame encoding and decoding. But the whole frame processing (effects, transitions) cannot be paralized. MLT can't do frame sequence paralizing, so it cannot calc the first frame on one core, starting immediately on the next frame on another core. Nope, unfortunately not.

My core i7 peaks at 30% when rendering a project, even with 8 threads. So I could render three projects in parallel. So unless you do a lot of parallal background rendering jobs, a core i5 seems to be sufficient. Mhz helps, but not more cores.