My current understanding is kind of like the link posted by roslinpl

https://bitcointalk.org/index.php?topic=268879.msg2974007#msg2974007

Meaning some motherboards do not supply enough power to the cards' ram. However some other motherboards (especially if designed for mining) actually do supply enough power and powered risers can be even detrimental to the results.

Powered risers do not work for these guys for instance, they are using a mining mb:

http://www.youtube.com/watch?v=WfqfEpanpig

In real life just see what gives you better hash rates.