Is it possible to use an SSD for the RAM preview in After Effects CC?
An SSD is a replacement for a hard drive, not a replacement for RAM. 

RAM previews are rendered into your computer's system RAM, not to internal storage (and then you can save the RAM preview to disk if you wish).

Having said that, I recently replaced a 7200rpm internal hard drive in my macbook pro, and added some more RAM. It makes a huge difference to the speed of loading and saving, and seems to improve render times to some extent too. It's like buying a new computer- I would definitely recommend it.