I'm not the 'support team' but I can answer you.
Yes, you can run x64 versions of Operating systems in Parallels.
I would recommend leaving at least 3 GB for OS X, specially if you have Safari (or another browser) open on the OS X side, or else performance will become very sluggish, this means assigning only 1 GB to Windows (Windows 7 works fine with 1 GB).
From my past experience of running Parallels with 4 GB RAM installed, the maximum amount I was able to use on Virtual Machines was 1.5 GB RAM (leaving 2.5 for OS X), by use I mean before OS X would become slow due to paging. This of course depends very much on what applications you have open on the OS X side, and how much memory they consume, that's why I mentioned Safari as it can easily consume 1 GB. If you close most applications on OSX side before starting Windows VM, then maybe you can assign 2 GB to it.
Last edited: Jan 30, 2012