Unlike the much more complete Wii undervolting "guide" this is just a smattering of info found when undervolting the DC, PS1 and N64.
Dreamcast info (8w to 5.5w)
N64 info (5w to 3.5w)
PS1 info (1.5w to 0.7w)
Like everything, use this info at your own risk and results will very likely vary...