I reckon the 3 sites (TechPowerUp, Tom's, HardOCP) quoted above have, more or less, established the upper bound (8%) and the lower bound (~0%) of performance loss for a GTX470 going from a PCI-e 2.0 x16 to PCI-e 1.0 x16.
To further estimate where in this range your particular setup belongs, I scan through the results and note that the higher the resolution (and AA/AF), the less the performance loss is. At 5760x1600 and 2560x1600, the loss is practically nil. But at 1280x1024 (which is closest to your 1400x900), the loss is around 2%. And given also that yours is a 470 (vs a 480), the loss should then be even lower to, 1 - 1.5% perhaps? Not a big deal.