So to do this you will have to be a windows user or the windows batch will not work ok to do this you need to make a windows batch file make it just like a the windows batch for bukkit but change it to this @ECHO OFF
CD /D "%BINDIR%"
"%ProgramFiles%\Java\jre7\bin\java.exe" -Xmx600M -Xms500M -jar minecraft.exe
you can change the 600 and 500 to anything but make sure you have the RAM to run minecraft with that much RAM and never have the Xmx lower then the Xms or it won't work like if i put Xmx500M and Xms900 or something like that it will give you an error and make sure to put BOTH minecraft.exe and the windows batch in the SAME folder! to see if it worked in minecraft 1.3.2 you can press snooper settings in your options and look for Jvm_arg and 2 the Xmx and Xms should match the one you put in the windows batch if not you did something wrong if it does work your good and if you want this to stay at the same RAM NEVER turn on minecraft from minecraft.exe and you will still need to make bukkit use less RAM as well if your computer has 2GB it's not going to be lag free but it won't crash and make sure to turn down your video settings if you have a 4GB or better you should have no lag and no crashes as it should use only 1.50GB in stand of 1.75GB or higher witch would crash on a 2GB computer