I know this is an older thread, but I didn't see where anyone definitively answered the question about the Optliplex 755/760 using Xeon processors.
From what I have read and what I have done, you absolutely can use any 3000 series 775 Xeon processor on these systems; no bios update needed. I have put a X3230 in the Optiplex 755, 760 and 960 as a direct replacement of either C2D or dual core pentium with no issues what so ever. So, I know for a fact that the X3230 works as a direct change out. Dell isn't going to tell you this anywhere in their spec sheets. I have another 755 waiting until I can get my hands on a X3380 to test on it as well, but I have absolutely no reason to think it won't work...
Z