I have the Asrock 775Dual-VSTA which also has only a 4x PCIe slot. While I've never tested it with a PCIe card, I have seen Anand's number which show on average a 10% performance loss when using the highest end cards. Slower cards have a less impact with loss and low end loss nothing, as they use less bandwidth.
I can say that my board is awesome. I've had another Asrock motherboard that was also stable as a rock. I strongly suggest Asrock.
I can say that my board is awesome. I've had another Asrock motherboard that was also stable as a rock. I strongly suggest Asrock.