Yes, you can use PCI-E x1 for the x16 video cards since the OpenCL/Mining applications aren't host<-->gpu intensive.

The caveat is that an x16 wide slot can supply up to 75W of power and the x1 slot is not designed to do that, so it's recommended you get powered risers (pulls the slot power from the PSU directly instead of through the motherboard) to prevent trace burnouts, smoke, etc.