Minecraft Java Arguments Error?

The current version is 1.7.4.
i have optifine and i've noticed that when i press f3 minecraft only has 400MB of allocated RAM. I try and allocate more RAM by editing the profile and ticking the java arguemts box and chnage the 1 to a 3 (i have 8GB RAM) but minecraft doesn't seem to work if the java arguments box is ticked, optifine or no optifine. Is it somthing to do with my java? I have tried re installing my java.

Which version of Java are you using?

If it is a 32-bit version of Java, I believe that you would be limited to 2 GB of RAM to run the Java Virtual Machine (JVM) no matter how much you have.