I really don't find Dell all that useful. I'd already been to their site. It's not bad for configuring a system but hell for finding parts. The only thing that comes up for optiplex 745 CPU's are some celeron's ! for poweredge 860 machines. Follow their upgrade wizard and CPU's aren't an option, just double-priced RAM & drives.
The sales people can get you answers but they are pretty careful not to reveal actual detail like CPU models as they want you to buy from them.
I just went back there and after another 1/2 hr decided they are useless for this task.
Most likely the E7xxx wont work being that they are 45nm. Dell would have had to update the bios for that CPU to work so unless you can find an updated bios you are stuck with the E6xxx duos, but to be honest with you there will be no noticeable difference in performance between the two processors.