Socket A mobos used the +5v rail for the CPU whereas newer ones use a seperate 12v plug to power the CPU (the 2x2 one)
The RAM is on 3.3v I believe
PCI = 5v & 3.3v (apparently older PCI slots = 5v, newer = 3.3v)
USB ports etc. will use 5v
Onboard sound will probably use either 5v or 3.3v as well