DELL is very protective when it comes to independently trading parts, or even buying aftermarket. I used to own a Dell XPS GEN I laptop, and I wanted to order the ATI MR9700 to ATI MR9800 upgrade. I have the skills to do the upgrade myself, however DELL, even after I spent $400 for the upgrade, essentially required me to have the technician come over. Even he couldn't understand why it was needed for him to come over when it was a simple swap of part for part, but he explained that in order for DELL's warranty policy to be satisfied, a technician has to do all the repairs, be it at home or at a depot. For some reason, DELL doesn't trust its customers, even its XPS customers (and now, apparently Alienware), to do upgrades by themselves. Part of the reason people buy high-performance computers is that they understand the requirements that go with them, and many of these consumers are quite adept at repairing / upgrading parts on their own.
For James, you can get physX to work on ATi cards, there is a workaround for it. It does require a download of this (for win7)PhysX workaround x64
orPhysX workaround x86