To the best of my knowledge, all AMD Radeon HD 6000 series graphic cards have PCI-e 2.1 interface not PCI-e 2.0 interface.
While all PCI-e 2.0 cards will work in a PCI-e 1.x slot, there have been a few complaints of a PCI-e 2.1 graphics card not working in a PCI-e 1.x slot. I think it is pretty rare, but it is something you should know in case the HD 6850 does not work with your motherboard.
You don't need to worry about a PCI-e 1.x slot bottlenecking a HD 6850; there is plenty of bandwidth.