Okay, I have two questions:
1: Is it vital that it be a laptop? You get a lot more bang for your buck on PC, to the point that I would only suggest gaming on a laptop if there is no other option.
2: What are the "certain reasons" that it must be Dell? You're cutting out all the major gaming laptop brands (with the exception of Alienware, assuming their status as a subsidiary of Dell qualifies them) by limiting yourself to Dell, which limits your options significantly.
Edit: Okay, I lied, three questions:
3: Where are you purchasing? Given your budget, I'm assuming either US, Canada, or Australia, but I'd need to know which it is to make a proper suggestion, as prices vary quite a bit between them.