External Clock is the FSB, which you should never put that high - generally 120 is the point of failure.
You overclock by changing the multiplier... If you're lucky, you haven't fried your USB, SATA, PCIe, and DIMM slots.
no, your thinking of modern i3's and i5's, this is a core 2 duo. multiplier is locked but you can change your fsb as high as your motherboard is stable. the fsb is separate from usb, ssata, pcie etc in socket 775. the fsb is 4x multiplier on these boards, so if its 200mhz in bios you x by 4 and its 800 mhz actual fsb.
Anyway, What you need to do is clear your BIOS with the jumper on the motherboard, you might need your motherboard manual to do this or it may be labeled on the board. This will get everything back to default and hopefully get your display back. you can then get back to tweaking. 65C is the highest i would go with temps on that cpu. I have had my fair share of overclocking c2d6600 and c2Q6600, they overclock well, you can add more vcore voltage to get it stable at higher frequencies, but if its not posting at all just reset the bios first, hopefully you havnt blown anything up.
No everything is currently fine (I'm typing from the computer I've OC'd), when i try to go past 375 i just get no display, but if i turn the comp off and turn it back on it automatically reset to 375 and everything is working fine.