Now you will think “Why not?”. Some time ago I read a blog post which said “If you are asking yourself if you want to install Office 2010 as 64bit version, you should install the x86 version”. Why that?

The reason is, that if you are asking yourself you worry about incompatibility with over software. And you are right! If you install x64, all add-ins you use have to be x64 as well. You should however install x64, if you need more than 2GB for Excel :-/ Or you use a SQL Server to store your data and install x86…

If you want to read more, take a look at 64-bit editions of Office 2010 on MSDN:

If users in your organization depend on existing extensions to Office, such as ActiveX controls, third-party add-ins, in-house solutions built on previous versions of Office, or 32-bit versions of programs that interface directly with Office, we recommend that you install 32-bit Office 2010 (the default installation) on computers that are running both 32-bit and 64-bit supported Windows operating systems.

Ishai Sagi mentioned that the ActiveX control which is used for the Datasheet view for SharePoint lists needs x86 as well!