Don't use thermal gel WITHOUT a heatsink, otherwise you'll have goop all over the place. This compound is intended to be applied in small portions only to create a better bond between your heatsink and components while protecting the chips from scratches and nicks that could cause damage.
Applying a heatsink/fan to your north bridge could lower the heat and therefor give you the ability to over-clock with stability while protecting you from frying the chip due to the higher voltage being applied. You should research your individual chipset/motherboard package to determine if this modification will actually help you at all.
Steve Benoit
Stable Technologies
'The way IT should be!'