I think SSD is a must for games (and OSs) nowadays, It changes everything and it's not so expensive anymore.
I have a 256GB SSD (C://) and 2TB HDD (D://).. and my OS + all my programs/games are installed in C, when it's full, I just move the heavy stuff to D:// Drive.

And you will see a huge difference in games, not just single player games, but MMOs too, of course not a FPS difference, but a load time, you'll load maps/textures instantly while on HDD it might lag for a few seconds every time you move to a new area.

On my first test with SSDs, I tested the difference in speed when starting up windows, my old HDD took 1:20 seconds to fully start windows, while the SSD took like 7 seconds (and those 7 seconds were there for the welcome animation)

( when buying one, make sure it's 6GB/s and not 3GB/s )