ahem...this card has not any power connector point
by won't start what do you mean ?? pc showing no display or even the cabinet lights are not getting on / cpu fan not spinning ??
all pci-e x16 ( regardless of version ) gfx cards are compatible with every pci-e x16 slot ( regardless of version ) - if it's not working check for a bios update on the motherboard manufacturer's website.
i have read at least one such report about a month ago here only. Even i feel GPUs should work regardless of what version the interface is but if OP can try the GPU in some friends pc or even some pc shop. Just to be sure.