Hi Kronos2008,
I am replying on behalf of Dell Vikram M.
Your computer is shipped with Nvidia nForce 680i SLI chipset and supports Intel Socket 775 (FSB of 1333 MHz) . For more information on this chipset, you may refer to link: http://bit.ly/o2HhcK So if you upgrade processor to Intel QX6850 it would work with the FSB of motherboard (1333 MHz).
Apart from the options above, all processors in the table below will physically fit into the socket LGA775, but not all of them are supported by motherboard of your computer.
As you mentioned that most of the applications you use are CPU intensive, you might want to overclock the CPU as well. When overclocking is not done carefully, it can lead to system instability, crashes, or data loss and could even destroy components. However, when overclocking is performed carefully, it can increase overall system performance significantly and pose a minimal risk to the PC system.
You may use the 'Nvidia nTune application' to access, monitor, and adjust system components, including bus speeds and voltages, with clear, easy to use control panels. You may check the system requirements and download the same from link: http://bit.ly/XHenx
For more information on this, you may refer to link:
ftp://ftp.dell.com/Manuals/all-products/esuprt_desktop/esuprt_xps_desktop/xps-720-h2c_owner%27s%20manual_en-us.pdf
Copy and paste the above link in address bar and hit Enter.
Please reply for further clarifications.