Nvidia Geforce 8400 GS (PCI)- easy ins...

I just installed a new Nvidia Geforce 8400 GS (PCI) card into my computer. According to the Device Manager, the card is there, but when trying to plug my monitor into the port like the instructions say to, it doesn't work. The monitor doesn't turn on at all, but plugging back into the old monitor port works,minus not being able to change resolution from the instructions saying to disable and uninstall drivers for the old card.

The BFG site said that a power source of 300 watts or more was required for this graphics card, so one of 450 watts was purchased and installed. But the card still doesn't work.

For my knowledge, my computer reaches all other specs required for use.