I have replaced an old graphics card in a dell dimension e510 with a 5970 and a riser. I also put in a 600 watt PSU. The computer starts and the lights come on but it almost immediately turns off. If I remove one of the pci-e power chords the computer starts but the graphics card obviously does not work. I have tested the graphics card in another computer. I have tested the riser with the old x300 and it works fine. I have tried using molex for one of the power connectors. I'll have to check again but I'm pretty sure the computer starts withought the riser in and all pci-e power connected... I have also tried the riser powered and unpowered.

I feel like I am out of options and I don't know what to do...

Edit: I should also mention that I was able to run the 5970 and a gtx 670 on my main gaming rig with a 750 watt psu.