A 64 Gb SSD for windows is still a nice upgrade to have, or if you can afford a 128 Gb+, you can also fit a few games on.
But for general use, putting a pair of F3s in Raid 0 will give you the best performance you can get from mechanical drives, and for really cheap. The 1 Tb F3s and 2 Tb F4s are dead cheap now, you simply raid a pair of F3s, and add an F4 to keep a backup image. Back it up once a month and you are completely fine, and you have plenty of space for all your games and anything else.
I currently have just 18 games installed on steam, plus lots more that I havnt downloaded yet, plus a couple on impulse. If I downloaded everything I have on there, I would exceed 256 Gb in no time, as well as all the non digital games I have on top of that on disks, I'm looking at over 1.5 tb worth of games, though I dont install ones I'm not playing.
An SSD for games wouldnt be able to store hardly anything, and you would have to constantly delete and install games you are currently playing to make the use of the limited space, which wears SSD drives down.