The answer to that is both.So it was the hougyaku (sp?) that gave chad and orihime powers, and not ichigos overflowing reiatsu?
The hogukyoku functions in a way that it alters reality into making peoples' wishes come true if it's possible. It thus works within a probability limitation.
So to apply this to Chad and Orihime, what the hogukyoku did wasn't so much giving them their powers, Ichigo still did, but what it did was that it effected the reality so Chad and Orihime would actually befriend Ichigo and therefore also be able to gain their new powers which they wished for. Since Chad and Orihime attended Ichigo's school and were of the same age group as him, it was thus highly probable this would happen. The hogukyoku made it just more probable to the point where it happened.
So what the hogukyoku does is that it increases the probabiilty rate of something happening to close to like 100% if a person wants it enough, but ONLY if it's ALREADY possible. It cannot warp reality from something impossible into something possible, the possibility must first be there and the hogukyoku simply increases that possiblity. This is why Rukia never regained her powers AFTER she got the fake gigai by Urahara. The hogukyoku sensed her wish to loose her powers and gave her the opportunity for it to happen, presenting her with the option of Urahara's fake gigai. Of course it's more complex than this, but I hope you get the gist of it.
Or at least that's how I understood it, so I view it a lot like quantum physics. I hope I explained in a somewhat understandable way. I am not surprised people are confused over its powers. Most people don't even understand basic probability and how it's applicable in their everyday lives (luck doesn't exist, for example, but yet people insist in believing in luck).