I have a Targus backpack. I've had it for a couple years and it's extremely comfortable and spacious. I love it. I think it would work well for you. Here's the link:
http://www.targus.com/ca/cases_notebook_backpacks.asp
PS. Mine is the first one listed. =) Oh, but the downside to this one is that after a while of overloading it the zipper things tend to break, so just invest in some extra zipper pullers (sorry for my ignorance as to what they are actually called) or key chains work great as well. Plus it adds a little splash of personality to it.