First, if you're going to buy the riser without molex 1x-16x, buy these: http://www.amazon.com/gp/product/B006DQRUZ8
, and thank me for your savings(I have 5 of those in my rig with 5830s, working fine for 2 months until now)
From what I know, the molex powered risers are for using more than 5 cards in motherboards that don't have a molex near PCI-E ports, like the one in the image. Most motherboards LACK
that connector, so you'll need risers for more than 5 normal
GPUs or you'll fry the 12V pins in the 24-pin ATX mobo connector.
By normal GPU I mean 5830-like power consumption. With 5870s I think it's max 4 with unpowered risers per mobo. With 5970s and any DUAL-GPU card you NEED
powered risers, or you'll fry the small 12V wires in the riser(they pull more power from the PCI-E slot).
With my Asrock Extreme4 970, I have 5 5830s with no powered risers, running fine.
With the riser length, I don't really know, and I can't find info online. You should buy the unpowered 1x-1x riser(http://cablesaurus.com/index.php?route=product/product&path=1&product_id=8
), and use it from mobo to the other riser. Then you should use the powered 1x-16x riser (http://cablesaurus.com/index.php?route=product/product&path=1&product_id=28
) from riser to GPU, to minimize heat and losses by wiring length.