[QUOTE=jhodder;721960] Vastro Lorde is referring to an evolution of a hollow who has eaten enough souls to take on a human shape, and gain power beyond that of a Captain. While Ichigo's Hollow has both the shape and power, he is still not a Vastro Lorde, because he has never eaten souls. As such, he is simply classified as a Hollow. /QUOTE]
It seems that hollows and shinigami are something like two sides to the same coin, and the hougyoku just makes the transition easier/more effective. A soul can get stronger one way or another. They can do it alone (via training, etc.) or consuming another's spiritual power. On one hand, there may be a possibility that consuming another soul is necessary for the hollow to classify as one or another, but it sounds like consuming a soul is just another way of raising spiritual power. Once they reach a certain level of power, they can do new things, like ban kai or compressing themselves down to a smaller form.
Also, I-Ichigo said he was Zangetsu. It makes the most sense that shinigami, zanpakuto spirit, and the hollow parts of a person are just different faces of the person's mind/personality/power. Some sides get more developed than the others; the shinigami separate the animal side and the hollows embrace it.
It's much more like schools of martial arts than different species. Everyone does very similar things for the same purposes (sonído≈shunpo≈hirenkyaku) . The details, names and flourishes are different, but it looks like a skilled practitioner can learn the other's moves with enough work. The hougyoku may just be spiritual steroids... One of the main aspects of 'enlightenment' in buddhism and hinduism is the realization that one is not really separate from everything else in the world, so it's a far smaller jump to assume that everything inside your own head is all the same person, even if you have arguments :P.