In order to run Rubymine under a 64bit jvm one needs to run the IDE from the bat file.

In the bat file which is located at C:\Program Files (x86)\JetBrains\RubyMine 3.1.1\bin add this line

SET RUBYMINE_JDK=%ProgramFiles%/Java/jdk1.6.0_25

and remove the IF statements
::IF “%RUBYMINE_JDK%” == “” goto error

Run the rubymine.bat file voila 64bit jvm.