|DO you have a speaker hooked up to the main board. If so do you hear the POST test beep when turning the computer on? |
If you don't then there is something wrong on your main board. Reseat all the power supply wires, memory and cards. If it does not help then you will either need to take it to a shop, replace the components (Memory, Exp. Cards minus the graphics because you already did that) one-by-one to find the bad one, or replace the main board all to gather. I usually opt for the latter because it gives me an excuse to upgrade.
Is this a new system? If so have you contacted the OEM's tech support? It might be under warranty.