Do you know what board your Optiplex has it in? That will determine the degree of your upgrade. I've worked on Optiplexes as an IT tech. I will say that Dell made them in a bazillion different configurations, but for the 745's. If you can give me the model of your motherboard, I can make a recommendation.
Now, 100% of the 745's I've worked with were LGA 775 platforms, which means you're stuck with the Core/Pentium/Celeron CPU's... single-core or dual-core Pentium's and Core 2 Duos. That core 2 Quad is many times more powerful than any Pentium D and will be much faster than even the highest-end Core 2 Duo. The Q6600 is one of the lower-end quad-core 45nm chips in the Core line. It's possible you can go a little higher on the rung with the Core 2 Quad chips, but I can't say for sure until I know your board. because it depends on the chipset.