64 bit then....job done...check with dell that the motherboard will see all the ram.
But, why get a laptop? You will have more horsepower for the same price with a desktop. Also, you can upgrade videocards / add more drives / add more ram use a much better screen with better resolution. Not to put you off a XPS laptop but one of my clients recently bought one (not sure what model so I could be way off) and the viewing angle on the screen was not good at all.
Maybe ask Dell for specifications of the screen before you buy...or I'm sure there will be someone here who has one and can confirm/deny what i've just said.