I would shy away from any that did not have a molex for power. Without it they have to pass 5v power from the PCI slot which it is using itself. Granted the PCI bus can supply 25w and the usb power spec is 2.5W (5v @ 0.5a) with 4 ports that 10w. I'd rather not risk buring out something on my motherbd.
What youl should look into more is which chipset is it using and keep an understanding that PCI is limited to 133MB/s between ALL pci devices. So dont expect 100+MB/s from an external HDD/SDD