Your not going to find pcie saturation anywhere as there is no way to get a real reading of it. The biggest bottleneck is going to be cpu before the pcie slot. No matter the card, if it be low end or high end, all will benefit with more bandwidth. But even high end cards like the 6950 and 570 only lose 2-5% from x16 to x8, and 10-20% from x16 to x4. http://www.tomshardware.com/reviews/pci-express-scaling-p67-chipset-gaming-performance,2887-9.html
This is a really good 3 part article examining the cards, the slot and the chipset.