I need help with RAM

[meatwadbia]

I have been working on this computer i received from my brother for some time now and am looking to update the ram. it has 3 different slots, one of which i currently occupied with 512Mb and i have a few other cards of ram i jacked from some of my broke ass PCs.

When i tried to install new hardware, turned off PC, put the card in, it worked for about 2 days than i got a no signal warning on my monitor. when i took out the card the computer worked fine.

Is there a step i'm missing or is it possible that i am not able to add more ram?

Thanks for your help in advance,
_the Meat_

 

[Dangman]

Specs of the PC would help.

Oh and it's not "cards" of RAM, it's "sticks" of RAM

Make sure that mobo can support that speed RAM as well as find out the max capacity of that mobo.