Recently I've made a few orders of USB Risers and I found something interesting with the 470/480 problem with risers powered by sata. There are at least 2 kinds of sata to molex cable adapters being sold with USB powered risers.
1. SATA POWER - These are not safe with 470/480, even underclocked and undervolted (not sure about 8pin cards tho)
2. SATA - These seem safe with 470/480 but only if the cards are underclocked and undervolted
I had the SATA POWER ones burn up/melt on me. The SATA ones still get really hot if they aren't undervolted. However if the SATA are undervolted and overclocked, then they, SATA, will only get mildly warm at most! These seem safe so far.
EDIT: sata power is 18awg and sata is 20awg