http://www.delidded.com/lga-771-to-775-adapter/
You have to get an adapter and mod the socket.
You will also need to verify that it is compatible with your MOBO
https://www.bios-mods.com/forum/Thread-OptiPlex-360-380-760-780-960-Xeon-LGA-771-E0-1067A-Microcode
This may require a BIOS mod.
secondly, verify that your PSU can handle it.
It can probably do it. In my experience, the Dell PSU do a good job.
Worth a shot if you ask me. Not a whole lot of money for the possibility of big performance gains. Just make sure you are not doing this on your primary computer, I can definetly see the possibility of something not going as planned.