If I can buy SSD, how do I best use it to improve video editing?
All computer performance optimization is based on understanding critical paths (bottlenecks), then removing or relaxing the bottleneck until there is nothing more you can afford to improve (possibly because there is simply no known widget, at any price, that will improve whatever else is on your critical path).

Some very common critical paths in video editing are


Reading video data from disk
Rendering video data (for display and/or for writing to disk)
Writing video data to disk


When you import video data into a project, rarely is it all loaded into RAM, though it could be.  If it is loaded into RAM, then buying an SSD is only going to speed up the initial loading of your RAM--a one-time benefit.

More often, video is buffered into RAM, but it fundamentally lives on disk, and thus disk speed becomes important as you play or scrub through your files.  If you have 1080p ACVHD stream, it uses approximately 28Mb/sec of bandwidth.  Modern HDDs can average 25MB/sec to 120MB/sec or more, depending on whether they are connected via USB2, USB3, USB3.1, SATA, eSATA, etc.  But if you need to load and display from 8 streams, it can try to read 28Mb/sec times 8 streams = 28MB/sec from the disk, but because the streams are likely scattered across the disk, it has to issue many seeks and buffer lots of data, leading to uneven performance.  An SSD is an advantage here because the seek time is virtually zero.  A single SATA SSD could easily handle 8 and perhaps 24 streams without choking.

However, the computer's CPU and GPU are typically designed to handle HD and perhaps quad HD video, meaning that they are going to begin to choke if they have to display 24 simultaneous streams.  But 4-8 should be OK.

To optimize your system, you should use a system profiling tool to see just how much disk bandwidth your system delivers under load vs. the calculated bandwidth of your video streams.  And you should measure how much video rendering performance your CPU/GPU can do vs. the estimated complexity of your streams (are they all just passed through a virtual switcher, or does every frame have scaling, translation, cropping, blur, sharpening, noise reduction, color correction, etc)?  You can then do some math: how much does it cost to close X% of the gap for each different gap you can measure.  Which dollars have the greatest impact on closing the gaps you can close?

You might find that indeed an SSD is just the ticket.  Or it might be that your GPU is really your rate-limiting step.  Most serious video editing workstations are a collection of many high-end components.  Having just one piece of the puzzle might only speed things up by 5%, whereas having all of the pieces, even if only half as good as the best, can speed things up by 50%.

It does take a little technical knowledge to profile and tune a system, but what it really takes is dedication to the process and good measurement tools.  Or enough money to keep buying bits until you can no longer see any improvements.