inertia, why do you want to use the full ATX PSU?
Not enough juice from a micro ATX (SFX) PSU?
I'm thinking about the same thing (if you see my thread Mini Micro ATX Case)... but I think the Enhanced 200W SFX PSU should do for whatever you may throw into the small PC-42.
What I am more worried about is whether I can fit a P4 in PC-42. At least in PC-30, I heard that one reviewer said that he doubt a P4, with its huge stock heatsink and all will be able to fit. The PC-30 PSU is also vertical, and sits on top of the mobo's CPU area. I think it will be the same for PC-42...