Help with Pentium D 805 CPU on P5WD2 MoBo

Hi everyone!

I've had my rig like so for almost a year now and still haven't figured out how to properly overclock it!

Here are my specs:
Intel Pentium D 805 @ 2.66GHz
P5WD2 Motherboard [Latest BIOS]
3GB G.Skill DDR2-800 [PC2-6400] @ 267MHz [4-4-4-12]
NVidia GeForce 7600GT [PCI-E]
120GB IDE HDD - Main
300GB IDE HDD - Slave

My problem is that every time I over try to overclock my CPU, I reboot and my PC shuts itself off... and then powers itself back on within 3 seconds. This isn't for only major overclocking because it does this for every setting I try it with, even minimal changes.

I'm not exactly sure what's causing the problem or if there's something I'm missing in the BIOS.

I don't want to fry anything so if someone could please help me, I'd be very happy! Thanks!