hello,
as far as i know, you don't need powered pcie risers, if your mobo has additional molex connection points to provide extra power to the pcie lanes.
i'm not sure exactly how your mobo's power rail is set up, but i'd say the mobo was designed to run sli/crossfire and the 2 molex connectors are to power 2 cards in that configuration.
so you could probably get away with 2 powered risers and 2 unpowered risers.
you'll need powered risers for the cards you're going to be connecting to the non 16x lanes.
this might also be of some use:
http://www.gobitgo.com/articles/1001/How-To-Correctly-Use-and-Install-PCI-E-Riser-Cables/