SharePoint Designer 2007 and SharePoint Designer 2010 on the same machine:
SharePoint Designer 2010 is a tool that comes with Office 2010 which is used by the developers and moderately sophisticated business users.
SharePoint Designer 2010 can be used only for SharePoint 2010 sites; it cannot be used for SharePoint 2007 sites, same way Designer 2007 can be used only for SharePoint 2007.
We can install both SharePoint Designer 2007 and SharePoint Designer 2010 side by side on the same machine.
If we want to install both in the same machine remember that both of them will need to be in 32 bit only.
You cannot have 64 bit office 2010 and SharePoint Designer 2007 on the same machine.
You can have them in 32 bit versions only.