"How much do I need?"
120G is pretty good. You want your system and programs on this and then you want all your games to start from SSD. And you are running fast.
Honestly. Putting a solid state in my machine was like changing two generations of CPU along with the Switch from DDR2 to DDR3.
We are talking read/write over 200mb/s (do not think you will reach the acclaimed 500!). Your programs just fly open! Adobe Premiere stuff in 6 Seconds! RPG's and no more loading screen...you name it