I'm building a new system before MS pulls the plug on XP. It will have a Core 2 Quad (Q9450). I'll use it mostly for business apps and video editing and compression. Should I put on the 64 bit version of XP, or not? It will have 4G ram, and I know the 32bit version cannot use the full amount.
 
From what I've read, this version runs 32bit windows apps with WOW. I don't know what the implications of that are. I do understand that the 16-bit version of WOW is not available for really old apps (but I suppose one could use a VMWare VM to do that).
 
Any advise appreciated.

Compatibility is actually more of an issue with XP-64 than it is with Vista 64.  A lot of companies are still dragging their feet on XP-64 drivers and releasing Vista 64 drivers.  The situation is better now than it was a year ago... but I still wouldn't recommend XP-64.  Stick with either the 32-bit version or go with Vista 64.