With current games at 30 - 40 GB, the question you have to ask yaself is how much SSD space can you afford ? For example:
500 GB SSD has 466 GB of actual space
Take of 15% for free space needed and you are down to 396 or about 11 games.
Is that enough ?
Or grab yourself an SSHD .... A 2 TB SSHD will hold about 45 of those games and will cost you $100
When you play a game say Assassins Creed 3, all the important files for AC3 will be moved to the SSD portion of the drive automatically after a few loads thereby giving you SSD like performance for loading times.... then when you wanna play say AC4, the AC3 files will be moved off and the AC4 files moved on.