I've actually never had problems with Dell's customer service, though I take good care of my laptops and have not had to use it for anything very significant. My last laptop was a Dell Latitude I got through my college, and it was a workhorse for over 4 years. It's still perfectly serviceable now, and I gave it to my mom when I got my current laptop, a Dell Studio XPS 16, which I've been happy with so far. I'm planning on keeping it for the next 3-4+ years and it's my only computer, so I wanted something fairly powerful, though I only really researched Dell laptops because my dad receives a good discount through his work. I heard good things about the XPS laptops in general, but of course they do cost more, and if you're looking in the $500 range it's really out of the question.

For gaming, I'm not sure how well an Inspiron 15 would handle current-generation computer games. Just be sure to read reviews about the computer's performance and the available graphics cards (which you'll probably want to upgrade).

Regarding your last question, I'd definitely recommend getting Windows 7 unless you're really strapped for cash. It is fairly good for Windows and it'll maximize your computer's performance (which will presumably have 4 GB of RAM). At least according to my comp sci major friend.