Redgie Boy

Hi, anyone can help me with this topic. It seem to be difficult to have reliable information about the Dell MotherBoard of an old (2008) XPS 720 Black Dell Tower. I currently have a E6700 Core 2 Duo running at 2,67 Ghz and I would like to upgrade it to a Core 2 Quad or even better to a Core 2 Extreme Quad but I am really in the dark about what CPU the motherboard will accept (not only the pinout) according to the Bios that is already in place. Unfortunatelly, Dell doesn't support this model anymore and it seem to be pretty hard to find reliable information about it and I am pretty sure it will be even harder to change/modify my Bios if needed to make the change of CPU. Anyone out there with real knowledge about this? What is the easiest upgrade I can make without having to spend too much time, money and effort? I have heared that I could not go over a Q6700 (Quad at 2,67 Ghz instead of a Duo at the same frequency) and I have read that some other were able to upgrade it to a Quad Extreme with no problem!!! (depends what is the definition of NO PROBLEMS). I hope to get info ASAP since I have to make a decision soon. Redgie PS I don't have the budget to change the motherboard or any other significant piece of material.