Joel Reyes posts about the various VSTO SKUs and what project types you get with what SKUs.
The basic principle is that if you only install VSTO 2005 SE you get add-in projects for Office 2003 and 2007 and code behind InfoPath 2007 forms (no 2003 or 2007 document level projects). To get our Office 2003 document projects for Word, Excel and code behind InfoPath 2003 forms you need to install either VSTO 2005 or VSTS. Support for Office 2007 document projects won't come until Visual Studio Orcas (the upcoming release of Visual Studio).
Also, Joel says there will be 8 add-in projects in Visual Studio Orcas for Office 2003/2007. Actually, there will be six for 2003:
...and seven for 2007:
In addition VS Orcas includes the following "document" or "form level" projects:
Excel 2003 Workbook/Template
Excel 2007 Workbook/Template
Word 2003 Document/Template
Word 2007 Document/Template
InfoPath 2003 Code Behind Form
InfoPath 2007 Code Behind Form
And it includes two SharePoint workflow projects
SharePoint Sequential Workflow
SharePoint State Machine Workflow
I think that covers it...