ram is not just ram is not the same ram as anouther stick unless they are the same maunfactorer, from the same batch (that confused me now.)
for a start there is loads of different types of ram, so I will stick with the main ones. Sd ram, DDR ram and DDR2 ram. There are different speeds for ram, and there are hundreds of different combninations of ram timmings.
I think you may have shorted something when you put the ram in. (You are meant to clear the pc of all electricity, (turn the pc off completly, psu and plug, and you have to press the on button a couple of times to clear the electricity in the board, (I have never done this once, always remember afterwards.))) Anyway back to the point, I think it is possible that somehow you shorted the motherboard, and it has gone.
I recomend testing each part in the pc, and seeing if it is the motherboard that has gone.
(sorry, but it is what it sounds like, from what you have been telling us)
Cisco CCNA, Comptia A+, 1/3 through CCNP