I have just configured my new black box - @XI dual core Athlon that is killer - & my first visit with WinXP64. Revit installed & runs flawlessly, as does my other app's. I read up on XP64 prior to ordering it installed & came to the conclusion that although there is no real benefit right now, there didn't appear to be any real downside.
Then I tried to install my system printers, all HP's. Nothing appeared in the "Add Printer" menu so I looked for good drivers at the HP site. The bad news: HP doesn't have any 64-bit drivers for any of it's printers, and there is some vague committment to having something in the near future.
My thinking right now is leaning towards bagging the XP64 & dropping back to vanilla WinXP. That, of course presents a new set of problems, not the least of which is having to obtain a fresh copy.
If there are any similarly surprised virtual colleagues that have enjoyed this experience and arrived at a different conclusion I'd like to hear about them.