North Bridge and South Bridge..

do they both get the same voltage?
I mean, If I increase the NB voltage (VDD) I'll have to get my SB cooled?

Suma.

It is a good idea to cool the southbridge as well. The vdd does not apply to it AFAIK but increased FSB rates will heat it up. Also good cooling of the southbridge will help out your maximum FSB rate.

Use a passive heat sink attached with thermal epoxy. It doesn't need a fan.