Yes I disabled c states an made sure windows was set to high performance the other thing I did was turn of speedstep in bios after down clocked the first time it didn't fix it so I turned it back onThe Optiplex 745 has a very compromised VRM. 3 phase which is actually OK in itself, but only 2 MOSFETS per. Due to the full multiplier steps on the QX6800 3.45GHz is about it. Heatsinks on the VRM Mosfets will of course help. If you look at CPUZ 3.45 GHZ is actually the average overclock on an MSI motherboard for that CPU. Only the 4 phase ASUS, and Dell Dimensions go higher. Even the 2 core X6800 struggles to go 3.73 due to the B3 stepping.
The XPS 200 SFF, and Dimension 9200C (same thing) came in BTX SFF and had 4 phase VRM. Those MB should take a G0 QX6800 to 3.73Ghz. The Dell MB will shut down at 1.6V. so stay below that some and see what you get.
Here is a Dimension E520 running 3.7Ghz. 51% single core, and 58% 4 core ranking is not too bad for a 2005 computer.
I have runs on the chart at CPUZ 3.99Ghz on that system. There's a link to one in my sig.
You might squeeze a little more out of it by underclocking the FSB with SetFSB, and add another multiplier, then bring it back up. But the MB from the multimedia systems is a much better place to start.
Due to the age of these finding one with good capacitors is not easy.
Why overclock a Dell? Because they're there. Millions of them in fact. Just about every Dell, HP, Gateway, Acer, etc. Intel computer for almost 10 years was a BTX. Many of them are being sold now as refurbished computers for less than you can buy an operating system license. BTX was originally a...forums.tomshardware.com
The issue with the Dellls downclocking the FSB has been seen before. Do you have C states disbaled in TS. Do you have a High Performance user profile set in Account manager.