Minecraft: Did I do this right?

As I was exploring some newly generated terrain on my survival world, my game started to lag horribly. I decided to allocate more RAM to the game through the launcher as shown here (http://www.minecraftforum.net/topic/1395030-tutorial-allocate-more-memory-for-minecraft/), but using 1G instead of 2048M (the launcher uses G instead of M). Now that I've done this, my computer's fan is running a lot even when not much is happening in the game and the game just seems to be a bit groggier than before(less FPS, sometimes lags like a server). It made me wonder, did I allocate more memory right, or did I do it wrong? Is this supposed to happen?