8GB is enough memory for all but the most demanding of games but if you stream your gaming sessions, run music in the background or play windowed with a web browser on a second display, then you might benefit from adding another 8GB. It's not a necessity, but it does make it to where memory is never something to even consider when you're operating your system.
Installing an SSD for your operating system is a huge day to day operating upgrade, but it is not going to improve your gaming performance per say. Faster map loads but it's not going to improve your FPS. If you're installing windows from scratch on that new drive, make sure to disconnect your existing hard drive to avoid master boot files being written to both disks. That would mean that you need both drives connected for windows to operate so when one drive fails, you're hosed.