Jonny:
One word, Intellisense. The complexity of doing almost any programming with today's technologies, like ASP.Net, is just too great to rely on a cool text editor alone. That used to be a bragging point for some hard core programmers but it just doesn't make sense any more. Those "hard core" programmers were not necessarily any good or very productive (I have managed a few of them in my career). You will be exponentially more productive, and will learn about the ASP.Net and DNN architectures with one of the true development tools like Visual Studio 2005 or 2008 or even the free tools from the Express family (which you can get here http://www.microsoft.com/express/product/default.aspx).
ASPX is the default file extension for the ASP.Net "basic" page, there are other extensions by the way, like ASCX for user controls. ASPX comes from Active Server Pages, the X I guess is for eXtended. Notice that there is basically no resemblance between classic ASP and ASPX, only the first three letters are alike and nothing else.
By the way, I don't consider doing HTML, even in a text editor, coding, just my opinion, I'm a Computer Science major.
Carlos