This is totally weird. My computer is working fine today until I decided to add 2 more gig of ram into it. After that, it won't even boot up, there is no beeping sound, and there is no signal to the computer monitor at all. The screen is totally dark, displaying "no signal". After I remove the extra RAM, and switch back to the original working configuration, the computer still won't start up at all. I checked all the cable, video card cables, unplug and re-plug in everything. Still, it doesn't help at all. The power supply is working, since all the fans are still working. I stripped the computer down to the bare essential, one strip of ram, one HD, video card.. still, the computer won't boot... does that mean my motherboard is fried?
I tried to clear CMOS, but it seems to make it worse, now the computer will start for 2 seconds before restart itself again.. and keep repeating doing that until I shut it off.
My computer setup is as follows:
Core 2duo 6420
OCZ 2P800R22GK: 2 Gb (the extra 2Gb of RAM that I tried to install today is exactly the same as the original one I had inside the computer before this incident
Motherboard: Gigabyte 965P S3

Disconnect everything from the motherboard. Remove all RAM, leave only the CPU. Try powering it up in that condition. Do you get any beep codes? (of course, a speaker must be connected to the front panel connector on the mobo.) You should get something that indicates a RAM fault, since no RAM is installed. If you do get beep codes, note the pattern - short, long, number, etc. and look them up for your mobo. Try adding things back in, one at a time. The old (good?) stick of RAM, then the video card, etc. and see at what point any troubles pop up.

If you don't even get so much as a beep code when only the mobo & CPU are installed, try removing the CPU. Power up the mobo and see if you get a beep code then. If you do get a beep code, put the CPU back in. If that kills the beep code, your CPU is probably toast.

If you don't get any beep codes at all, either your speaker is dead (VERY unlikely) or your mobo is indeed fried. Do you have another mobo you could use for a reality check?

For some weird reason, my computer case don't have a built-in speaker (case: Raidmax Sagitta), there is no cable that I can attach to the speaker pin on the motherboard. But I notice something, after I remove everything (including cpu), I was able to power up the computer without it restarting every 2 seconds. But the screen is still totally dark (after I put the video card in to check).. what can i do now?

You should be able to get a speaker for cheap. Rip one (carefully) out of an old case. Hook it up to your mobo - you need to hear what it might be telling you. But. . . at least your hardware is doing something different, eh? Encouraging? Don't know enough yet. Your $80 mobo could be OK while your $300 video card could be toast. . .

somehow my computer came back life, i thought the PSU was toasted so I bought a new one. With my old ram in place, I was able to boot up my computer. As I thouhgt I have fixed the problem, I placed the new RAM in the Channel one again.. same thing happened once again. Computer failed to boot up, with no signal to the monitor, and it didn't help with the removal of new ram either. I then took out everything, insert the new ram into Channel 0 this time, with nothing in channel 1, using the OLD power supply, the computer boot up fine, strangely, after replacing the new ram with my old one into channel 0, the computer stops working again. To make long story short, I finally got the computer boot up again with my old set of ram, since I Got tired of trying to insert the new ram and having my computer died on me. I decided to stop it. I asked the OCZ rep, they told me to leave the old ram in place, go to bios, set voltage to 2.1V DDR800 and make sure 2T is listed, reboot, power off and insert the new ram in, then see if the computer will boot up. Here is my question, since I never OC my system (hence I never play around with the BIOS).. where can I go in BIOS to change it? (in MIT .even with ctrl+F1...I couldn't even find the place where I can change the voltage of ram to 2.1V, unless they go by some other technical term, and the place I can check whether 2T is selected)

Many systems require you to unplug the power supply from the wall and then plug it back in to do a new system check after an upgrade. Hence the reason switching power supplies worked. Put all four gigs in then unplug and try to see if it works. It should. If not you will need to look around your bios for dram voltage and increase it. Most likely unplugging PSU will be the key.

99% of the time this works.

What roadrunner197069 said.

FWIW, if you just do a "Start > System shutdown > turn off system", the PSU is still supplying partial power to the motherboard. It is essential that you physically switch off or unplug the AC cord before you do any work on the guts of the machine. If unsure, shutdown the system and then disconnect the AC cord.