I was having a similar problem with my P5Q Pro.
It would start up after a CMOS reset, allow you into the BIOS, but then wouldnt boot up after you saved the correct settings (although the settings did save). So you'd have to remove the power cord for 30 seconds or so (till the light went out) and then it would boot up normally.
If you tried to restart the computer via windows, it simply would refuse to boot again. Even if you turned it off via windows, and then on again, it wouldn't work.
Since i have upped most of my CPU/RAM settings past their requirements and it seems to be ok. Turning off C1E and speedstep was one thing i was told to do. After setting the NB voltage to 1.3v, and SB to 1.2v, as well as enabling load line calibration, i've had no problems any more (fingers crossed).
For the record, im running a Q9500, 8gb DDR2 (although i have 4 in atm just because i dont want to mess with it incase it stops working again!), 1x4870 1gb, 2x HDD, 1xDVD-RW, 5xcase fans in Antec 902 running from a BeQuiet 550W PSU.