IIRC, Q9400 has 1333 FSB, 2.66GHz and 8x multi which should have the RAM running stock at 333*2=666MHz. Since you have PC2-6400 per your sig it should be able to run at 400*2=800MHz. Assuming the CPU will take it, raising FSB to max the RAM should take you to 3.2GHz.
What settings have you adjusted beyond stock and to what values?
I just follow the oerclocking guide in this forum. When I tried to change the dram frequency and timing and reboot, it doesn't have the beep sound. I have to turn it off and reboot it. It is overclocking fail during post. If I set the those two to AUTO, it would boot fine. Right now, I am only OC it to 3.0GHZ..
From your sig it sounds like you have memory like mine. I never could get this memory to faster timings than the default 5-5-5-18. However I am able to run it at 900MHz (450 FSB).
If you have a Q9400 at 3.0GHz then I assume your FSB is at 375? Assuming you have adequate cooling you should still have a bit more overhead on the FSB although you may have to up the voltage a little bit. Max VID per Intel's spec sheet for the Q9400 is
0.85V 1.3625V. You should be able to get a close estimate of what the AUTO setting is applying by running CPUID although you'll have to take into account vdroop which will cause the voltage displayed to be slightly under what your BIOS setting is.
Yep, same RAM as mine. Based on my experience and my research in various forums when overclocking after my build I suspect you'll never get it to be stable on tighter timings. However you do still have some FSB room to play with.
I recommend using CPUID to find out what CPU voltage the "AUTO" setting is applying just so you can manually control that in BIOS for further overclocking. I haven't used your mobo before but most AUTO settings put CPU voltage close to the max (1.3625V for your chip).
With adequate cooling on the Q9400 and keeping the voltage down to prevent waste generating extra heat you may be able to do the following:
FSB = 375 * 8 multi = 3.0GHz (I assume this is what you're running now)
FSB = 400 * 8 multi = 3.2GHz (may require setting voltage into the ~1.275-1.325 range)
FSB = 425 * 8 multi = 3.4GHZ (likely will require upping voltage a bit more)
Ok, the 7500 should be able to provide decent cooling to bump up the FSB a bit more. Grab CPUID, the latest RealTemp (at least v2.90 last time I checked) and Prime95/OCCT/IntelBurnTest to keep an eye on voltage and temps and you should be able to start incrementing FSB and testing.
Dominator RAM allows for tighter timings (4-4-4-12 or lower) than XMS2 (typically 4-4-4-12 or higher) but it's up to you whether the price premium is worth it... GB-for-GB it's a lot more expensive. Plus slightly lower RAM timings rarely make much difference in real-world performance on a PC, although it's a great bragging point for folks on the forum
The nice thing about the RAM you've got is even if you run it at stock speeds your CPU should still be able to attain a nice OC... plus it's cheap.
Set your memory speed ot 800MHz or a 1:1 ratio and timings to SPD/Auto Vdimm memory voltage to 2.0V (higher than stock but under the Corsair max of 2.1V) and leave it there and check that it stays set that way until you finish determining how fast you can get the FSB/CPU speed to run. Take the memory out of the picture until you are finished with the CPU. Then you go back and figure out the memory settings. If you manage to reach an FSB of over 400MHz you would then need to again be concerned about your memory. You are trying to do two things at once, dont. Determine what the CPU will do, then determine what the memory will do, then you find a combination of CPU mulitplier, FSB speed and memory mulitplier/ratio that, typically because CPU MHz are more powerful than memory MHz, will run your CPU as fast as possbile and run the memory close to its specifications.