I think you already made up your mind, but dual core is the way to go. I currently build computers for some of my friends and I offer dual-core only so they don't come back to me in 2 months and get ****ed that I got them a single core.
The $100 difference may seem a little harsh right now, but in a year, or even half a year, you will be seeing games and programs that take advantage of dual core! It's worth it!