Ben, Intel played a mean trick on enthusiasts when preparing their Core 2 Quad Q9300 and Q9400 CPUs from the same family for launch. This may totally ruin overclocking attractiveness of the Q9300 and Q9400 solutions. The thing is that Yorkfield processors, unlike their predecessors from the Kentsfield family, work with 333MHz FSB. And it automatically lowers the default clock frequency multipliers.
Thus, Core 2 Quad Q9300 with the nominal frequency of 2.5GHz featuring a 7.5x clock multiplier. It means that you will have to significantly increase the front side bus frequency in order to achieve any noticeable results during overclocking. For example, in order to clock Core 2 Quad Q9300 at 4GHz, which seems to be quite attainable for this processor theoretically, the FSB should be increased to unreal 533MHz. I said unreal, because only a few of the existing mainboards can work with quad-core processors at super-high FSB frequency like that.
Numerous overclocking experiments show that the maximum FSB frequency contemporary mainboards can reach when working with quad-core processors and traditional cooling systems is 460-470MHz. That is why the typical overclocking result for Core 2 Quad Q9300 will be around 3.4-3.5GHz. Further frequency increase will be limited by the mainboard and the chipset, but not by the CPU that can definitely do better.
Overclocking quad-core processors by raising front side bus frequency is slightly different from the same overclocking approach for dual-core CPUs. It is in fact much more complicated. It is not enough to increase processor Vcore to ensure that it will run stably at high FSB speeds. To ensure stability you need to increase other voltages, too: CPU PLL Voltage, FSB Termination Voltage and NB Voltage.
By manipulating secondary voltage settings, you can push the maximum FSB frequency a little bit higher. Depending on the quality of the mainboard (Rampage Extreme etc) it could be really good if the mainboard allows increasing CPU PLL Voltage, FSB Termination Voltage and NB Voltage significantly. There is one thing you have to keep in mind during overclocking like that through: increasing these voltages will inevitably lead to higher heat dissipation of the chipset North Bridge. You must keep the Northbridge and southbridge cool.
Warning:
If you intend to use quad-core processors at over 460-470MHz FSB frequencies, you need not only to carefully pick your mainboard, but also modify the chipset North Bridge cooling system. By the way, this is when you could really use the mainboards where chipset heatsink can be connected to the liquid-cooling system, such as ASUS Maximus or ASUS Blitz, for instance.
In other words it really takes a great board to exceed 480 x 7.5 and as good as the ASUS P5Q PRO is, it is not the greatest overclocking board for a quad core with a 7.5 multiplier. Try incrementally moving up to 480. If it gets unsteady raise your CPU voltage not your memory, but make sure your Northbridge is not reading too high.
Curious is that the PRO Turbo by chance? That would be a better OCer and more stable at speeds of 480 and higher
Ted
Almost left this out. This is the ASUS P5Q PRO OC Guide
http://www.computerlounge.co.nz/forum/Default.aspx?g=posts&t=50
