here is some links
this one is for figuring out which cpu fastesthttp://www.behardware.com/articles/778-2/giant-roundup-168-intel-and-amd-processors.html
this one is info on Core 2 Duoshttp://en.wikipedia.org/wiki/Intel_Core_2
this is 745 user guides http://support.dell.com/support/edocs/systems/op745/en/UG_en/index.htm
this one is for 755shttp://support.dell.com/support/edocs/systems/op755/en/index.htm
also go on ebay and look up each model and see what came from Dell in those units
From my research I found that E6600/Q6600 will fit 745s though I havent confirmed that yet
I believe that almost any C2D/C2Q will fit 755s (E8xxx,Q9xxx etc)
I like my 745 actually even though it is a Dell LOL
It is quiet, draws low power and was CHEAP buying it used.
The business class Dells (optiplex,vostro and precisions) usually
are decent from my experience.