I bought a palit gt640(NOT GTX!) 1 GB graphics card and it worked fine for two weeks but after that.my computer went off and then it was not working anymore..So I thought that the problem was the power supply.I bought a bigger power supply.but again nothing happened.The fan of the graphics card works fine but nothing happens...My PC doesnt even beep in the start.The computer works fine with the onboard card though...Any ideas?