I can only add that I have been running 64bit Vista for many months now (not sure exactly, but probably over 6).
Have not had any problems, althouth I had to wait for Zonealarm to release a 64bit compatible version. Even the SDK (so far) runs fine.
As we all know though, no two systems are the same, for NW it is clearly wise to stay on 32 bit. Unfortunately for the rest of you, it may be a gamble. You'll never know until you try, but that costs money. Damn hardware and software manufacturers....