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...
Dreamcast info (8w to 5.5w)
N64 info (5w to...
Dreamcast, PS1, N64 Undervolting info (incomplete)