Do you have a system speaker installed (one that beeps at POST)? If so, are you hearing 4 beeps? This is the Video Card error signal. If you don't have a speaker installed, you're probably not hearing the 4 beeps.
You don't have all the power connectors plugged into the card. The card needs 2 power connectors installed to work properly. Your card should have come with an additional power adapter that you plug into two molex from your PS and then plug into your card, in addition to the PCI plug that your powersupply currently has.
