So basically you are asking about upgrading XPS 720 right?
This is from the manual http://support.dell.com/support/edocs/systems/xps720/en/OM_EN/appendix.htm#wp1057813
Intel® Core™ 2 Duo (dual-core processor)
Intel® Core™ 2 Extreme (dual-core processor)
Intel® Core™ 2 Extreme (quad-core processor)
I see that QX6850 is routinely installed into XPS 720, I also seen confirmation of QX9650 installed in XPS 720, but this requires more investigation. If you are interested in QX9650 then read this http://en.community.dell.com/support-forums/desktop/f/3514/p/18908121/19031107.aspx
For now it is official chart below
Here is another looking similar Dell's validated CPUs
PW016 Kentsfield Quad QX6850 3.00GHz
RX656 Kentsfield Quad QX6800 2.93GHz
CX696 Kentsfield Quad QX6800 2.93GHz
UP673 Kentsfield Quad QX6700 2.66GHz
C864D Yorkfield Quad Q9450 2.66GHz
HU585 Kentsfield Quad Q6600 2.40GHz
MP626 Kentsfield Quad Q6600 2.40GHz
Numbers to the left are Dell part numbers.
I hope it is enough information for you.