Here is an official comparison of the different versions of Vista.
You can, of course, upgrade to Vista to the Ultimate version that has everything. Here is a Vista upgrade calculator. For me, it would cost $159 to go from Home Premium to Ultimate.
Right now, I don't need the FAX functionality. I primarily use send2fax.com to send and receive FAXs electronically. But I also have several computers running with older versions of Windows that have FAX software and have two multifunction printers that can also FAX (though I only really use them for scanning and copying).
Rather, if I do end up upgrading to Vista Ultimate, it will be to have more control over the security settings, be able to back stuff up en mass, be able to host remote desktops, and synchronize files on different systems.
I should note that some of these differences are only visible when you start digging into the documentation. For example, I found out about the remote desktop stuff and synchronization when trying to set them up. I am currently using Synchronize It! to synchronize files among my different computer systems. The problem is that it is becoming laborious to make sure that they are all up to date. The Vista synchronization facility looks like it might be useful for me - if it were available. It isn't in the version of Vista I have (Home Premium).