Two PCI-E cards - only one is recognised

This was my setup: gigabyte motherboard: GA-970A-D3 with an AMD Raedon 6670 in PCIE slot 1. running DVI cable to monitor and HDMI cable to another big TV (extended display)

That all worked fine.. then I went and built a flight simulator that sits next to the PC. The sim has two screens, one big HDMI TV and a small DVI monitor. So I went and purchased a second video card an AMD Raedon 7770. and put it in PCIE slot 2. Now when I first booted up the computer I could see both cards listed in the device manager. I did not have any monitors connected to the second hard at the time - so I thought good. I turned it off and finished the cabling only to find that now only the new card was working. Nothing changed inside the PC. The 7770 card is the only one that would display anything, I tried the VGA, the HDMI and the DVI of the 6670 but nothing would work. I removed the 7770 card and the 6670 worked just like it normally would. put the 7770 back in, and only the 7770 would work.

I read the mobo manual, and was a bit upset, I found although the board has two PCIE 2.0 16x slots, the top one runs at 16x where as the bottom one only uns at 4x! so I thought it made sense to put the more powerful card into the 16x slot. after doing so, still the same issue, only the 7770 had anything on the display.

In my bios I don't see any options for enabling or disabling the second slot. so I don't think its a bios issue, plus as I stated it did at one point work.

So my question is how can I get both cards working at the same time?

the only one thing I can think of is, I have a cooler master 500 PSU. and the system is an AM2+ 4100 bulldozer. one SSD, 2x HDD, 1x DVD, 2x 4GB RAM. According to my calculations I should only need 380watts, but could it be possible im underpowered to power up both video cards? the 6670 doesn't have an external power input, just motherboard, where as the 7770 has an external 12v connection. I am not sure though why the system would even start if it were underpowered and why the 7770 always get the power regardless of what slot it is in.