Don't worry about worrying! It's a natural psychological reaction after you spend a great deal of time doing research, and then make a decision between two products that are extremely similar.
I looked at the 8600 today after Dell started its new deal, even after I just purchased an Acer Aspire. I did feel a little bit of regret, but at least I know that what I have has worked out for me. I have had two Inspiron 5000s that gave me no technical problems, but the build quality on both was very poor, and I made a promise to myself that I wouldn't buy Dell again before trying out some other brands.
So I stuck to that rationalization, and I'm happy with what I have. I picked up the on-site 2-year warranty extension as well, so I've got some peace of mind. (Plus, there is a coolness factor in having something different, as most people I know who have laptops have Dells. Some are very, very happy, others have complaints, mostly with CS.)