You do not need 12GB of memory for Minecraft or Starcraft II. The performance benefit over having 8GB will be 0.
The highest memory footprint I've seen a single game use are the current Alpha builds of Star Wars: The Old Republic (yes, I'm an Alpha tester, have been for 6 weeks and will be testing until released). Its commit charge for the game's process is ~3.5GB.
8GB is plenty for games. Period. If you need more, it'll be cheaper to add it in the future, since memory prices go down, not up.