RSColonel, surely you are not implying that I'm going to be productive.

RSColonel nailed it. The Ultimate/Professional editions usually include Windows built-in Windows features that are handy (in my case Fax/scan, backup tools), so you don't have to go hunting for 3rd party software to do it.

The biggest consistent difference I've found between Windows Home User and Professional/Ultimate editions are administrative tools. The home editions don't have them at all or are defeatured. If you like to setup a lot of administrative policies for users, programs, or the OS itself, the professional editions are a must. For example, I welcome family members and roommates to use my comp, but I don't want them going through stuff. This type of thing becomes even more important when you are on a network, running a server (including hosting games), or allow access using any remote connection tools. In a nutshell, these tools let you micromanage how your machine behaves and what is accessible.

Here is a quick comparison, if we can deduce anything from Vista
http://www.microsoft.com/windows/windows-vista/compare-editions/default.aspx

My suggestion was little nonchalant, I ass-u-me-d that with the cash you were spending on the machine you might as well just buy the full package to avoid missing out on something you might want later. Truly, if you're not doing anything fancy, don't bother with tripped out security settings or the like, Home premium will be just fine. If you know a student who could it for you for like $20 on campus, I'd go the ultimate route -- otherwise the increased cost might just be a waste for you