Sorry for late reply. for some reason I don't get notifications though subscribed.
I've asked over at frozencpu whether I can purchase single atx-cables for the modular PSU's in their product line, and described why, but they haven't replied
The thing I worry about the most, with just splitting up the atx cable into 2+ is that if e.g the 3.3V can draw say 5 amps at most, I probably won't be able to get in to the most efficient range of the PSU before hitting the ceiling on amperage.
A loose, but not too far off guess, is a use be each node at around:
CPU @ 35W
RAM @ 2.88W, 1.6A.? datasheet(pdf)
I'm not too sure on how to understand the specs for a total usage from the set.
And something for the MB which probably isn't that much.
So probably around 50-100W pr. node (except a 'main' node with DVD-burner and harddrives.)
But I might be worrying for no reason.