Is a PCI/USB card able to draw (enough) power via the PCI slot to run something like an ext hard disk?
The USB ports must comply with the electrical specification for the USB standard, irrespective of how or where the port is implemented (mobo, PCI, PCMCIA) - so yes, if the device is designed to rely only on the USB port for power, and it conforms to the relevant spec, then it will work.
Some devices actually have 2 USB sockets so that they can get double power.
It gets a bit confusing when you have multiple devices on a single USB root hub, as the spec. defines the power to the root hub, IIRC*. So a device may have enough power if it is the only device on the root hub, but not if there are other power demands by other devices on the same root.
This is not the same as a USB hub, BTW - that is a downstream splitter from a single USB port that may or may not inject its own power supply.
SD
* I may be mis-recollecting this - please correct if so.