Just from personal experience when building my last gaming rig... I have an SSD for the OS & programs, and a 1TB standard drive for general files and data. I made sure to get the 7200rpm drive, for a little speed kick, but otherwise it's nothing special. As far as general operation, having Windows on the SSD was a massive performance boost when compared to my previous build, which was similar in almost all other respects. The SSD just lets your operating system fly faster, so you feel much less lag and hesitation compared to an HDD-only configuration.
So, just to test some things out, I initially installed my Steam library on the SSD, and played with the whole setup for a while. Impressive speeds, good load times, etc. Then, on a whim, I moved my Steam library to the hard drive and played some more. I was actually surprised that my load times stayed very fast, there was not an appreciable increase between having my games installed on an SSD vs an HDD. This could be due to a few factors (like not having to access OS functions and game functions on the same drive, allowing some parallelization maybe?). Either way, I've kept the games on the HDD, which has an added benefit of not feeling restrictive on the space overhead of the smaller SSD - I can keep more games installed without constant install/remove cycles.
Bumping from 8Gb to 16Gb of RAM also made a huge impact in overall performance and gaming in my new rig. I found games loading and zoning faster and cleaner, less stuttering when running tasks in the background, and overall the system was much snappier.