The 12v molex connectors are the one that go to the IDE HDD, Optical drives and gfx cards like the 9800 pro (at least I think they are 12v IIRC).
If the HDD accepts the SATA plug/connector, I'd be inclined to use it. My concern about too many things on one connector is that I'd be afraid of the possibility of corrupting the data on my HDD.
Sorry, I'm not sure about dynamic power alotment. But I have seen peeps put too much on one line and have probs, usually with the gfx card. Thier solution was to limit how many things were on the line.
If you don't feel data corruption is likely or a prob, you can try hooking up the new fan. If there are no probs in gaming etc, fine.
Seems all of my PSU's have 4 lines with the 4 pin molex connector. As a matter of practice I use one exclusively for the gfx card