In short, here is Microsoft's word on who should use 32-bit Office:
"Important The 32-bit version of Office 2010 is the recommended option for most people, because it prevents potential compatibility issues with other 32-bit applications, specifically third-party add-ins that are available only for 32-bit operating systems."
And here is the word from Sage on 32-bit Office:
"Currently, Sage ACT! does not support the 64-bit version of Office 2010, only the 32-bit version. However, Sage ACT! and the 32-bit version of Office 2010 can be successfully installed and integrated on the 64-bit versions of Windows Vista®, Windows 7, or Server 2008 operating systems."
As more programs become available in 64-bit, I suspect ACT! will too. For now, we must wait ;-(