We've been patiently eagerly awaiting the release of 64 bit versions of Revit. This represents the removal of the barricade preventing access to additional RAM. Large projects will benefit from this primarily and obviously only those using a 64 bit operating system like Vista or XP as well as hardware that permits more RAM.
It is only available for subscription customers at this time. (too bad I can't access subscription)
Don't expect increased speed, it makes more ram available to Revit but does not inherently change how Revit processes data (this is a technically inept statement, don't expect giant strides in performance...just access to nearly as much ram as you can afford)
It works side by side with 32 bit Revit on a 64 bit computer as the 32 bit version is installed in a special folder for 32 bit applications "Program Files (x86)".
You can open files with either version. The Revit application is changed to support 64 bit not the file format itself.