+AMAL SB You definitely want to add an SSD to your system, if the budget allows.
Advantage: Windows boot faster, games load faster, system much more responsive.
I used to recommend to people that they get a big enough SSD for Windows installation and applications, and then to place their steam library to a HDD. I recently had to temporarily use a HDD for my steam library after a Windows rebuild. I re-downloaded my steam library and noticed that it took much longer to allocate the space on the drive, before downloading each and every game; on a SSD it only takes about two seconds. The thing I noticed is while allocating space, the download pauses for that entire period. It was really very annoying, and something I had never noticed before, but the SSD is so fast that I never perceived the pausing of steam downloads in-between each game.
Anyway, consider purchasing a Samsung 850 EVO (2.5") ssd. They are great.