Op, it's pathetically easy to add a drive, it's plug and play as far as windows is concerned, quick format to get it recognized and you are done. Moving drives is/can be a pain, and can take a good long time depending on size of data to be moved. And God forbid if you forgot to install the new drive as a boot capable drive before moving over. That puts windows on a storage drive instead.
What I'd do is as you said. Keep the 120 solely for OS and OS necessary stuff, like Java, office, steam main folder etc. Install all your games to the 256Gb. The only thing the game will add to OS drive is the saved game files, which aren't large.
The only things I'd add to that, is remove hibernation on OS drive, totally. Hibernation automatically portions 75% of your total ram size for itself, and never gets used. Hibernation is for laptops, your pc will wake from sleep just as fast as hibernation. Most modern pc's that's 12Gb of ssd space you get back.
Also, make sure to img the OS 1x a month, keep it on the hdd, even a second copy on the 256 ssd, just in case. One good virus can wipe out the OS drive, simple to just repartition/format and install a prior image. If the OS was on the 256, you'd end up scrapping everything and an image of that drive would be massive.