I see only a three phase regulator on that motherboard.
http://www.pcplanetsystems.com/abc/images/big/asusmobo21big.gif
Place a finger on the MOSFETs and coils, near the CPU socket.
If they're getting hot, don't overclock too much further.
If pushed far enough, MOSFETs have their own SOA and version
of thermal runaway.
I have one system here, that when I overclocked it, the
regulator circuits near the CPU socket were getting too hot
and I had to back off.
*******
When you overclock, you generally need a bit more VCore to do it.
Does your motherboard support VCore adjustment ?
I checked the manual, and don't see an adjustment for it.
You might need a "volt mod" for VCore, or, figure out how
to change the VID code being sent to the regulator.
You would also need to research, how much additional VCore,
is safe for a Prescott. I don't have a number in my head for that.
On an enthusiast motherboard, VCore can sometimes be adjusted
at the BIOS level (boost type setting).
VCore voltage, and max frequency, go hand in hand. You will
get a certain amount of overclock, without adjusting Vcore.
To go further than that level, you need more VCore.
http://benchmarkreviews.com/images/reviews/processor/655k-oc/LGA1156_Overclocking_CPU_Frequency_vs_Voltage.png
On my Asrock board, I did a volt mod, and that boosted the
voltage when I needed it. The voltage regulator in that case,
had an offset feature, so the mod was not too hard to do.
Paul