VI's run "inside" Pro Tools and will be limited to the max amount of RAM that PT can use. VI's that stream samples will benefit. VI's that can load their samples into RAM and then stream from RAM will benefit. ReWire VI's will benefit as they will be running completely outside of PT. PT will still benefit overall because it will now have a full 4G of RAM to itself and not shared with the OS etc.
The big thing for myself with W7 64 is the massive RamDisks I'll be setting up.
Shane