1) Yes, 3x2GB is a good idea. Do keep the triple channel enhancement at all times if you are not changing motherboard.
2) GTX 960 gives great performance for the price.
3) Particularly for WoW you have to consider how many random reads occur for your style of gameplay. Your style of gameplay will determine that. When you first load the game, enter a new zone, walk into a portal, major city, battleground, raid or anything else that will make the game load new textures a lot of random reads will occur. This is due to the game’s file structure. For example, if you have seen your character (or other people) on underwear while the game loads their armor’s textures (for seconds or minutes), this is one of the main reasons why it happens; your storage cannot keep up with the gameplay’s demand. There’s a difference for someone that is constantly on the same place running daily quests (ergo using the same textures) vs someone that is doing raids or battlegrounds. If you do raids and battlegrounds then you want an SSD. Think of all the textures and effects that are loaded on a raid. If someone casts a spell who’s texture wasn’t previously loaded, a seek and load will happen, you have to see the visual animation, hear it and watch it’s the desired effect occur. Multiply this by the number of characters, how fast it all happens in the game and how big textures are.
… plus the overall bootup and performance gain for Windows and every other software you run.