Where's king? He got a q6600 in his Optiplex and he did the BSEL mod on it. He's got a OEM Dell Pc overclocked to 3.2ghz stable lol. OP you can wait for him he knows everything about upgrading Dells but to my knowledge q6600 is the highest you can go. Those you listed are extreme versions and cost more than the new SB platforms you might as well do a mobo, ram and cpu upgrade then
Thanks Dadiggle
But actually it is an E4300 1.8 800mhz pin modded to 2.4 1066mhz
Just bought a E4500 and will try to get 2.93ghz with pin mod
With that said
The 745 only offficially supports E2xxx,E4xxx and some E6600
they have to be 65watt, 65nm and no higher than 1066 FSB
I have read threads across the web stating that a Optiplex can support
q6600 BUT considering they are about 150 to 200 USD right it
really doesnt make sense to buy one for an Ole Dell Opty

Dadiggle is right
at that price might as well do new build
You could almost do a X 3 Rana mobo/cpu/ram/psu for that amount
I did get my E4500 for $50 shipping included so
I figured why not
I can end up with an Optiplex 745 with a 3.0ghz (2.93) Core2Duo
I love making OEM pieces of crap do things they were never meant to do LMAO
Hopefully I dont have the magic smoke leave my cpu....
they dont work right without the magic smoke in them LOL