yes the 12v is the main one for the cpu itself in most every P4 and Athlon/opteron motherboard since the slot athlon/p3's i believe, but the 5v and 3.3v are still important for video cards (overlocked especially), memory (pulls quite a bit if you have 512 or 1024MB of the junk overclocked, watch out!), sound/nic/addon cards, etc.
hehe