I don't know how SLI GTX 275 compares to a GTX 680m or 670m. And I don't know about PhysX.
Click the below link for a review of the GTX 680m. Benchmarks are towards the bottom. Be aware the benchmarks are based on laptops with different configurations so actual benchmark results can vary between different laptops. For example, the GTX 680m may be tested in a laptop with quad core i7 CPU, while the Radeon HD 7870m may have been tested in a laptop with a dual core i7 CPU.http://www.notebookcheck.net/NVIDIA-GeForce-GTX-680M.72679.0.html
In the middle column is a list of mobile and desktop (for a relative comparison) graphic cards. Desktop card are in black text and the GTX 275 is not listed at all.