Hi John,
ArcGIS for SharePoint is a SharePoint solution and I believe it is x86/x64 agnostic. ArcGIS for SharePoint v1.1 installs on SharePoint 2007, which has both x64 and x86 installs available. So I think if you install ArcGIS for SharePoint v1.1 on your SharePoint 2007 site it should work.
Hope this helps,