ST61G4 NOISE and HEAT

I built a ST61G4 for a client and they have been complaining about the
noise, so last week I replaced the 80mm fan with a nexus and cut out the
restrictive grill in the back.
It was a big improvement but then the Northbridge fan was the loudest
component. I had a zalman fan controller and reduced the speed on the 40 mm
fan as much as I could until the fan started making a pronounced clicking. I
then unplugged the 40mm fan altogether and the machine is really quiet.
The Northbridge is recording between 47 and 52 deg C. originally the temp
was ~37 degC
My next step is to remove the fan from the original heatsink and check the
temps.
My CPU (p4 2.8c) is running nice and cool at 32C.
I would like to put a larger heatsink in it but I don't want to glue it on.
I understand that gluing is the only way to put a Zalman on.
Does anyone have any ideas on a passive heatsink that uses the hooks at the
corners.
Also does anyone have any experience with the temp. of the NB and at what
temp I should be getting worried.
Thanks Doug