The differences are sometimes as simple as performance and age of the components. The i7-2600 based system is a newer CPU and MB design and probably more expensive than the i7-960 series. Below is a side-by-side benchmark comparison of the two CPUs from Tom's Hardware. As you can see, the i7-2600 CPU clearly outperforms the i7-960 on all tasks especially video encoding. I suggest that you wait for the XPS 8300 (i7-2600) if you can.
http://www.tomshardware.com/charts/...compare,2417.html?prod[4786]=on&prod[4474]=on
http://www.tomshardware.com/charts/...compare,2417.html?prod[4786]=on&prod[4474]=on