Hmmm... that's strange that the HD 4670 wouldn't work on the board. It's PCIe 2.0... the standard. Unless the board is faulty, it should work. I've read about several folks with late Gigabyte boards having PCIe slot problems. In fact, it happened to me with my Gigabyte board. I had to RMA it for a new one.
The Asus 1GB Radeon HD 5670PCI-E 2.1 on the otherhand is PCIe 2.1 Some folks report problems with 2.1s in 2.0s slots. Personally, I've never had any issues with any PCIe version in any board. But I hear it exists.