Well first as Chugot9218 explained the RAM doesn't mean that much when it comes to FPS in games as much as the CPU and GPU do. What kind of computer are you running? Maybe I or someone else can suggest an upgrade to boost your FPS. As of now with no hardware upgrades a way to boost FPS could be to,
1. Make sure you are on the latest version of Minecraft.
2. Turn down your graphic settings, maybe turn off smooth lighting and turn down the render distance a bit.
3. Get OptiFine! OptiFine is a mod for Minecraft that boosts your FPS and makes your game run much smoother, many people get up to DOUBLE the FPS when using this mod.
Hope this helps!
~Upstagebuffalo