No monitor display

have hp pavilionmedia c m8100ypcw/tv/pvr. Had hard drive fail. replaced hard drive and uploaded vista and had very bad vga display. ( nvidia geforce 8500 gt installed) updated driver and cpu would freeze up on start up. rolled back display driver back to default vga and would work but bad picture ( had lines throughout the screen like old nintendo graphics). replaced power supply with 650 watt, and nvidia gtx 260. when cpu was turned on, no display. reinstalled the older graphix card and had same bad display, put the gtx 260 back in and no joy. changed cards out again and now get no display at all for old card. motherboard ??????