Nope, with this level of GPU there will be NO performance difference between 16x PCIe 2.0 and 16x PCIe 1.x.
The only difference between the two standards is that PCIe 2.0 gives twice the bandwidth and twice the power through the slot.
As such, a 16x PCIe 1.x slot will give you the same bandwidth as an 8x PCIe 2.0 slot.
With that in mind check out this review: PCI Express And CrossFire Scaling
As this review is testing a much more powerful 5870, your GPU will have even better scaling.