500W for ram / cpu is extreme maybe 2 cpu's also 10GB Ethernet ports is likely to be build in.This does seem like an "Apple-ish" version of modularity... A lot of flexibility, but Apple maintains control.
One issue with the idea of passing 120V power between modules is the possibility of building a configuration that won't run from a standard outlet. The base unit is going to need to support at least 500 watts for CPU and RAM, especially if the W3175X rumor is right (and it seems like a very likely CPU). High-end GPU modules could easily draw over 300 watts apiece (the GPU itself can draw around 300, plus power supply overhead and anything else bundled in there). I/O and storage modules are lower draws, but they aren't negligible - a unit with a couple of 10GB Ethernet ports, multiple SSD slots, etc. could use 100-200 watts, and anything with PCIe expansion needs to provide 75 watts per slot, even assuming the slots don't support GPUs, which live in graphics modules.
One CPU/RAM module (500), one high-end GPU module (350), and one dual-PCI I/O module (350) is already a 1200 watt potential draw. That's barely going to fit on a 15 amp 110V circuit (probably not complying with best practices about spare capacity), and it'll severely restrict what can share a 20 amp circuit with it - a couple of monitors or a laser printer could trip the breaker. Add a second GPU module, and even a 20 amp breaker is awfully close! A dual-CPU configuration with the RAM slots full probably won't fit on a standard 110V 20 amp circuit unless it uses a single lower-power GPU and is relatively light on storage and PCIe.
If modules had separate power input, people might plug too many into one breaker, but at least Apple isn't encouraging it...
The top HP-Z has an 1450W PSU but that is likey to high end of an system for apple to go.