Interesting to hear how it works out for you.

I have watched this video where they put together a 6x 280x rig with some insane hash rates. They  use non-powered risers but each card adds significantly to the draw from the motherboard, i think around 50W actually (that board was designed for mining though). They end up using 3 power supplies.

http://www.youtube.com/watch?v=WfqfEpanpig&feature=youtu.be

So i might give powered risers a try. Anybody can recommend good suppliers in the US? I looked on newegg but what they sell gets bad ratings...

I have some decent old 650w power supplies around, without pci-e 8-pin connectors. I wonder if something like this would work, to add to the 1000 that i already have:
http://www.newegg.com/Product/Product.aspx?Item=9SIA29716C8626