HD 4870 - Black Screen

I have just recently purchase the HIS HD4870 1G and try to install it on on pc with the following specs

CPU - Q8200
Mobo - GA-EP45T-UD3R
RAM - 2G Kingston DDR3 1333
Hard Drive - Sata Seagate 360G
PSU - Antec True Power 750W

(nothing overclocked)

I have uninstall the old video card driver 9400gt, insert the HD4870 into the PCI exp and plug the 2 x 6 pin power cable and tried starting it up.

I start the pc then the psu makes a weird dragging air sort of noice follow by some wield beeping sound from the case.

After few secs i think things seems to run and the fan on the HD4870 runs. No image on the monitor says nothing dectected, cant even see the bios startup.

Then try detaching all hard drive (to avoid any driver issues) but still no display at all. I tried putin the 9400Gt with hard drives unplug and it boot with display and into bios.

I am unsure if this is the hardware problem on the pc or what is affecting it. I reinstate the 9400GT and the computer runs perfectly again.

I appreciate any help and thanks everyone in advance for responding to this thread