If you self-publish, you won't get the distribution and publicity that comes with being represented by a reputable publisher. Even if your book is available on the web, you have to let people know about it. It needs to be in catalogs in bookstores. It needs to be sent to reviewers. You have to build a buzz. And the inexperienced author cannot do that for himself.
Trust me, you can't do what they will do for you, especially if you have a day job. They have contacts. My publisher's wonderful, wonderful marketing rep got me on the radio, got me interviewed by the New York Times and Salon.com, got it reviewed in national trade publications, got me amazing opportunities I could never have gotten on my own--and I had a platform and was writing about a very hot topic at the time the book came out.
If your book is good enough, you'll get an agent and it will sell. It will.
I can see the paradigm changing in the future with ebooks, and in many ways I hope it does, but
right now that's the way it is. Brutal truth.
I would recommend reading up at
Absolute Write and
Writers Beware's Blog--they will steer you in the right direction. There are some scammers out there waiting to take advantage of you, so be careful, and best of luck with your book.