Rendering video is CPU intensive.
It doesn't use much RAM or hard disk etc.
Increasing your RAM will help while editing video, since more can be loaded into memory, which is faster then your hard drive. It will NOT help when rendering the final video.
I agree that a SSD is one of THE best upgrades for any computer, since it will speed the system up. I don't mean speed it up so slightly that you can't notice, you WILL be blown away by the speed difference, it really is that huge. This is because its not using any mechanical parts to read and write data, it uses chips, similar to RAM.
However, again, this will NOT speed up rendering very much, if at all.
Rendering uses CPU cycles. The faster your CPU, the faster your render.
Some software can use your GPU as well as the CPU to render. It seems like the software you are using allows this.
Hence the biggest speed increase you will see in your rendering will be obtainable by either upgrading your CPU or your GPU. Personally I would do the former before the latter.
Following this, a SSD, as mentioned above, will give you an all over general system speed boost!