I would get USB riser cables myself because they use individually shielded twisted pairs for data, but if you are dead set on cheap ribbon cables then I would get the x1-x16 powered version. Keep them short though, 7" or 19cm is standard IIRC. Longer ones may work, but like I said pci-e communication is a balanced pair of data lines. The longer (or wider, as in more pci-e lanes) they get, the higher the chance the noise can't be "comp'ed" out. If you get to that point, you'll have erratic mining behavior, or no mining at all.

http://en.wikipedia.org/wiki/Twisted_pair