Well, technically if you're asking about Management Studio Express vs Management Studio - it really has nothing to do with which database you're using on the backend unless you're trying to manage features that SQL 2005 Express just doesn't have. I use SQL Server 2005 Management Studio Express to access and run queries against SQL 2005 Standard databases. If you're talking about Express vs Standard SQL, then Matt's link will help you.
Here is a link of what you cannot do with the free version of Management Studio Express. Do you need to access some of these features, and thus what's why you need the standard edition of the Studio? The standard edition of the Studio comes with SQL 2005 Standard, so if you've purchased this from TechSoup, you should already have the installation material.