Hi dLTPN,
Optiplex 745 has three variants: Optiplex 745, Optiplex 745C and Optiplex 745 FLX. Intel Pentium 4 HT, Pentium D, Celeron D, Intel Core 2 processors are only tested with your computer by Dell. You can check the processors tested with Optiplex 745 in an image below:
Optiplex 745 support LGA775 socket. Q6600 Core 2 Quad processor supports the same socket. However, the max TDP of the same is 105W. This might work on your computer. For more information, you can check the comparison of both the processors from link: http://intel.ly/12TdMFh
Dell does not offer processor or motherboard upgrades for purchase as discrete items, nor does Dell recommend upgrading the processor or motherboard in any given system. Therefore, Dell cannot support systems in which the motherboard or processor has been upgraded through third-party services or products. For more information, you can refer to the link: http://dell.to/TXOJja
Please reply for further clarifications.