I used XP x64 for some time about six months ago, and didn't have any issues with any programs including CNX1.x. But as I didn't see any improvement in performance and never needed more than 2GB RAM (had 4GB installed with XP x64), I returned to "normal" 32-bit XP and 2GB RAM. C2D E6400 CPU btw.
I don't have Vista 64 but friends do/did, and NX2 crashes thier systems regularly. Based on the tracebacks the problem is in the drivers (video and SCSI). So my guess is the problem isn't necessarily the OS itself, but highly system dependent. It is also possible as the Vista drivers get better the problems will go away (although both me friends chucked Vista). It is always a good idea to look for new drivers.