Hi Mamiamato
What method did you try when allocating RAM to minecraft? Did you enter Java Runtime Environment Control panel, or did you make a .bat file? If you used the JRE Control panel, i can tell you that it didn't work for me either. BUT when i used a .bat file i had no problem.
You can make one by Right clicking your desktop and choose New -> Text document. Call it whatever you want and open it. Inside the file you type:
javaw -Xmx1536m -Xms768m -jar "[path to Minecraft.exe]"
Then go to File -> Save as... And set filetype to "all filetypes", change .txt to .bat and then it should work!
Just run the .bat file instead of the minecraft.exe. That should start Minecraft with 1.5 GB RAM and leave the remaining 1 GB to the system. If you want you can allocate 2 GB by changing it to:
javaw -Xmx2048m -Xms1024m -jar "[path to Minecraft.exe]"
To see if it worked, open a world and hit F3, then look in the top right cornor and see how much RAM is allocated. It won't always be exactly 2048 MB but something more than the default 910 MB
Hope it helped
As a little bonus you can add:
"@echo off" (without quotes)
at the top and add your username and password after the "path to minecraft.exe" to make it auto login