Ab Aeterno stated it above fairly well. Just to give a little extra information on why it may not actually be a complete waste, if you are using mods for minecraft then minecraft is going to need more memory to deal with the extra stuff.

If you are running minecraft with about 2GB you should do fine but if you are using mod packs such as tekkit with that then the 2GB will not be enough and your game will crash as it runs out of memory. Just something to keep in mind if you are considering getting more RAM.

Oh, thanks .

ram really isn't that important when it comes to FPS.. it's more for speed.

People buy more RAM because they hear it on forums and from their friends, who most of the time haven't got a clue what they're talking about. RAM (Random Access memory) is like short-term memory, where your PC stores important memory for running programs. So if you're low on RAM, buying more RAM will obviously speed your PC up (and get you more FPS). The reason adding more RAM doesn't get you more FPS (in most cases) is because you already have enough RAM to handle a specific game, say 4GB, which is what it's allocated to use. So if you add another 4GB, you're now going to have 8GB of RAM. But that game only needs 4GB, so the extra 4GB you just added doesn't make any difference. But then people hear that the reason their computer is slow is that they haven't got enough RAM, so they go and buy it anyway. You FPS primarily comes from your GPU (graphics card), which is what you should be most interested in for gaming.

As for how much you should allocate to MC, I'd recommend at least 2GB, although I personally use 4GB (but I have a load of mods installed).