Norton Anti-virus doesn't work on 64-bit - they say it does but it doesn't. I had it running on 32bit vista just fine but on 64bit, the license manager hangs right after the install.
You can't use viiv... It's only available in 32-bits.
Also, regarding viiv - even on the 32bit version, it has to be installed before you upgrade to Vista as intel doesn't have a clean-install, only an upgrade version.
That being said, I went 64-bit and it's great. Everything else has loaded and run just fine. I started out with 32-bit and reinstalled the 64bit version after I verified that I had all of the HW drivers. I picked up an additional .75G ram (went from 3.25 to 4!). Also my raid drives are much faster, average access went from 13.7ms to 10.2ms... YMMV, depending on the rest of your HW.
Good luck.
Update on Norton - Of all of the vendors in my system... Norton executes out of the data area. This was causing a DEP vioation and Vista just shut it down. Hooray for MS; shame on Norton. However, it would have been nice if one of them had mentioned why before just shutting down the DEP problem - oh well, Norton 360 does run on a 64bit system (I've got good virus and firewall prtection - one less major concern).