you have three levels of memory involved in a game, HDD>RAM>cache.
Everything is initially on the HDD, and when you get to a loading screen or enter a new zone (anything that needs new data) your system brings all the necessary data into the RAM (as much as it can, depending on you RAM size), which is much faster than your HDD. It's stored here until it is time to be used, when it is brought into the cache (very small but VERY fast memory used by the processor) to be executed.
This is why low amounts of ram makes things run slower, since it has less room to put things and will have to draw from the HDD more often. Adding an SSD only makes the transition into the RAM faster, and once it's in the RAM the speed of your game is not affected at all by your SSD.
That said, SSDs are still worth it in my opinion. They make boot times insanely fast, transferring data is a breeze, and they are very safe in terms of failure. So i'd encourage one if you want to spend the money, but know that it wont make your game experience much different outside of load times, which isn't a huge deal. It certainly won't help your framerate or anything.
With games you will notice the biggest difference in load times.
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