[KarmaKiller]

Hi, welcome to TechIMO.

1. PCI-E 16x is compatible with PCI 2.0. Same slot, same cards, no issues there
2. Check for a BIOS update.

I'm pretty confident in saying it's nothing to do with the GPU itself.
My money is on the Power Supply. We need thay make/model. And if you could list the specs on the side sticker, that would help. Even though your PSU says "700watts", there is a BIG difference between a quality PSU that pushes 700watts, and a generic that pushs 700watts. Even with that said, the specs clearly say if your wanting to run both GPU's onboard the x2, you'll need at least a 750watt PSU.

So post the specs of the PSU. And check for a BIOS update.