How can i set my Bukkit Server Ram?

@ECHO OFF
SET BINDIR=%~dp0
CD /D "%BINDIR%"
"%ProgramFiles%\Java\jre6\bin\java.exe" -Xincgc -Xmx1G -jar craftbukkit-0.0.1-SNAPSHOT.jar
PAUSE


This is my Serverstart.bat configs.
I set " -Xincgc -Xmx1G" this Part like this " -Xincgc -Xmx2G"
But Cmd says " Error occurred during initialization of VM , Could not Reserver enough space for object heal,Could not create the Java Virtual Machine"
....
I have 4GB RAM. How can i do ??