It seems Sun Microsystems have begun supporting 64bit versions of Windows including 64bit Firefox and 64bit Internet Explorer as of the 8th of December 2008 
The JDK beta release has the 64-bit plugin, You can download it from here: Java Platform, Standard Edition 6u12 Binary Snapshot Releases
Edit:
You can test your 64bit Java with your 64bit browser here: Test whether Java is working on my computer - 1.5.0, 1.6.0
The other tools the JDK installs can be separately removed if your only after the plugin

The JDK beta release has the 64-bit plugin, You can download it from here: Java Platform, Standard Edition 6u12 Binary Snapshot Releases
Edit:
You can test your 64bit Java with your 64bit browser here: Test whether Java is working on my computer - 1.5.0, 1.6.0
The other tools the JDK installs can be separately removed if your only after the plugin

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