I know I say this a lot, but use Magic Launcher. You can modify the RAM allocation from the GUI. You can also have it run Minecraft in a larger window, or even fullscreen by default.

hell, ill try it it keeps on saying javaw is not recognized even though the video is telling me to do exactly that.
i keep on getting an error
*** Starting Minecraft ***
Minecraft version: 1.2.5
Minecraft.jar: minecraft.jar
Java path: C:\Program Files (x86)\Java\jre7\bin\java
Memory: 3072 MB
LauncherPath: /C:/Users/derek/Documents/MagicLauncher_0.9.8.exe
Error: Could not create the Java Virtual Machine.
Error: A fatal exception has occurred. Program will exit.
Error occurred during initialization of VM
Could not reserve enough space for object heap
whats the problem here?