Dell Optiplex 755 Mid-Tower Upgrade Problems

I have this Dell Optiplex 755 Mid-Tower, Win 7 64bit, Core 2 Duo 3.0 ( E6750 chip ) CPU, 4x GB Ram ( PC5200 @ 667mhz, 1x GB chip per slot ).
I upgraded to the Core 2 Duo 3.0 ( E8400 chip ), and 8x GB Ram ( Samsung PC6400U @ 800hrz, 2GB chip per 4x slots ).

I first turned off the Desktop, unplugged all wires from back, opened-up case, removed old Ram Chips and installed new, then removed CPU Heat Sink to remove old CPU, then installed new CPU ( and used CPU Thermal Grease ), and reinstalled Heat Sink. Then installed all cables & wires in back...I left side case open to view any issues during start-up.
So, the problem I have now is that the Mid-Tower turns on, but makes sequential noises...Heat Sink Fan ( Big Black Fan ) turns on for a couple seconds then turns off, then the Vid Card Card turns on ( fan spins ) for a couple seconds then stops. The front lights also keep blinking on and off ( lights on the CD Rom ) several times in sequence. I get no Video through the Monitor ( Monitor light is on ).

I keep uninstalling-reinstalling the CPU & Ram in different order ( ex; New CPU with Old Ram, Old CPU with New Ram, etc, etc ), but still the identical sequential noises arise...I even tried to put back my Old CPU & Old Ram, and still having the same problem. I just turned off the Mid-Tower, and uplugged everything...Figure I try and get help here first before taking it to a computer store.

Any help would be appreciated.
Thanx, Joe
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More about dell optiplex 755 mid tower upgrade problems
  1. I would check very carefully that you have all the cables plugged back in correctly and that you didn't accidentally loosen a wire internally when working in the PC. Especially since it now does the same thing with the old components back in. Did you remember to plug the cooler back into the header marked CPU FAN and not some SYS FAN header by mistake? Try it with just one stick of RAM in the first slot.
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