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.