Do you use any of office through another program (Automation?) - if you do, you need 32bit.
(Although I am using Win7 x64 on a machine with 12GB RAM, I am still using Office32 as I have various applications that interact with Word and Outlook.)
If you are up for it, install Officex64 and check to see if any of your regular programs are borked, if not - keep it on, otherwise revert to Officex86.