8800gt no output

I have the following

asus p5k mobo.
4 gig ram.
150 gb raptor HDD.
1 optical drive ( asus again ).
q6600 quad croe cpu.
Corsair 620 psu.

I am unable to get the 8800gt to work. system powers on but no output form the card. Ive swapped it for the 7300 I had lying around and the sys works fine with this.

I rma'd the 8800gt I had and got a replacement same thing happens I swapped it for the 7300 and at power on no output to my monitor.

On both occasions the card had power and no beeps to indicate a failed post. As the same thing has happened with both cards I think it is something other than a duff card.

I thought it was the psu but other threads have suggested that it should be sufficient for it.

If it is not the psu not being man enought can anyone suggest one that whould run it. If the Psu should be enought does anyone have any ideas on what the problem may be???

I don't want to be insulting here, but are you plugging in the power dongle that goes directly to the card? If I'm not mistaken the 7300 doesn't require external power, so it'd be easy to overlook when upgrading.