The main board for the Dell Optiplex 755 can be found on Dell's support page and you can get additional information from their support forums.
Thanks for joining the community.
I understand you are looking for processor compatibility on Dell systems.
Please bear in mind, this type of compatibility is provided by the board maker, in this case, Dell. Your system integrates Intel components but the actual maker is Dell.
If the Optiplex computer integrates an Intel Q35 Chipset, please take a look at this URL for compatible processors:
On the left hand side "Compatible Products"
I hope it helps.
I apologize to both of you; my mistake. I see that the 755's board IS a Dell, not an Intel. I'm still looking for the manual/tech. info. for it--in Dell's site. Meanwhile, thank you for your help. I've just checked your reference, and it doesn't look like that quad processor is suited to Dell's board; it isn't listed among the compatible processors.