I can assist you with overclocking your QX6850.
I can assist you with overclocking your QX6850.
I had a task I needed to complete. You can find the details in my profile link for Overclocking Dell BTX Computers to gather some suggestions.
You haven't mentioned your current MB.
CPUZ for QX6850
http://valid.x86.fr/top-cpu/496e74656c28...303047487a
CPUZ for QX6800 displaying multiplier overclocks 15x266 fsb. My model is #5, it was #2 when I shared it.
You should begin by addressing the CPU heat spreader. Use progressively finer sandpaper on a glass surface to achieve better metal-to-metal contact with the heatsink, which should already be polished if quality exists. Investigate which TIM compound is most effective at the moment. Apply only what's necessary. Aim for the thinnest layer possible, avoiding trapped air. This means placing a small dot in the center that expels air as it spreads.
VRM cooling is recommended if not already present. Ensure air flows in through the cooler and out through the case, covering the VRMs as well. Keep in mind that air doesn't exit straight from fans; it spreads in a cone shape with a significant gap in the middle unless restricted. Many small adjustments contribute gradually toward achieving 4GHz performance with these CPUs. Water cooling is ineffective below room temperature. Increasing RT air through the radiator will help, but the CPU must dissipate heat effectively and the power supply must deliver sufficient watts for these older units.
You might consider an LGA775 Club socket at OCN. They have a strong preference for these.
http://www.overclock.net/forum/8-intel-g...-club.html
This is the link to my motherboard
https://www.gigabyte.com/Motherboard/GA-...-rev-10#ov
4GHz stability seems unlikely for that CPU. The QX9650 should handle it and even better, and an unlocked model can too. With the unlocked 65nm chips, the faster FSB leads to larger multiplier steps. The 45nm variants use half-step multipliers. Since this board supports 1600FSB, you can set the CPU to around 3.67GHz and gradually increase the FSB until you hit thermal limits under load. Higher voltage boosts speed but also increases heat. You must match your cooling solution with your CPU's needs. A good cooler should have enough radiator space; however, the ribbed hoses and rough water block will restrict performance compared to a custom loop. Both water loops and air coolers push heat into the air, which can only get you closer to limits—not below them. A 3.8GHz stable is likely your final target, possibly a bit higher. Make sure the fan circulates air effectively around the CPU and VRM area, as it's easy to overlook this with a water loop setup.