Having only PCI-e myself this is something I've looked into. FWIR it depends on the card. IIRC there weren't many cards that took advantage of the full bandwidth of PCI-e x16 (2.0) at the time, and thus should have enough bandwidth on 1.0 x16 to not bottleneck. I think I looked at this early in the life of pci-e 2.0. (2007-8?)
More info:
Quote:
PCIe 2.0 motherboard slots are fully backward compatible with PCIe v1.x cards. PCIe 2.0 cards are also generally backward compatible with PCIe 1.x motherboards, using the available bandwidth of PCI Express 1.1. Overall, graphic cards or motherboards designed for v2.0 will be able to work with the other being v1.1 or v1.0.
http://en.wikipedia.org/wiki/PCI_Express