I've been quite in a slump recently about deciding if I shall go for a x2 500GB SSD setup since my HDD is dying. One for OS and the other one for games, now I realized that I got a lot of outdated stuff and it's about to get really pricey so I really really shouldn't go that overkill. I decided to go with a WD Blue 1TB and a Crucial MX200 SSD 250/500GB, now I can't decide if I shall use the SSD as most do for OS because of the loading times even though the loading times on my dying "Hitachi HDS721075KLA330 750GB" don't bother me at all. And I am not sure if it really would make any difference using it at games.
Your OS on an SSD will make a bigger noticeable difference than having games on the SSD. Your not always playing games on the computer, but 100% of the time your using the computer, your using the OS. SSD boot times and even in-OS tasks feel snappier, it is quite a nice experience in comparison to any hard drive. That's my recommendation, its fairly common to setup your computer this way and having done this on a couple machines now I cannot recommend it less.
I can't really tell you if its worth it too you, thats a you decision, but I can give you my arguments for each. 250GB is a lot for programs and your OS, if you keep it to only that and maybe only your favorite games, you won't run out if your good at managing. If you get a 500GB, you won't have to touch it for a long while and never swap in a larger drive realistically if you don't just leave crap on your drive. You can also accommodate some games, not all but some. Idk its really a you decision, I would probably just go 250GB just because I really manage my data and I have used less than half my 120GB Pcie SSD's capacity used after the better part of a year on this install. But then again, I really keep it minimal.