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"The socket type on the OptiPlex 745 motherboard is a 775. Processor options for this model included Intel's Celeron D, Pentium 4, Pentium D and Core 2 Duo lines. The lowest option was the Celeron D 300 series with a 533 MHz bus speed. A Core 2 Duo with 1066 MHz bus speed and two cores was the most high-end option. The other processors have 800 MHz bus speeds."
Google is the easiest way to find these answers...
"The socket type on the OptiPlex 745 motherboard is a 775. Processor options for this model included Intel's Celeron D, Pentium 4, Pentium D and Core 2 Duo lines. The lowest option was the Celeron D 300 series with a 533 MHz bus speed. A Core 2 Duo with 1066 MHz bus speed and two cores was the most high-end option. The other processors have 800 MHz bus speeds."