I have a new Gigabyte-X48 an the North Bridge has a large heat sink.  Currently I'm OCing my Q9450 at 3.4  (FSB 420) while keeping the voltage stock.  I want to add a Mini Kaze 40mm to the NB heat sink before I increase the FSB and the voltage.
 
There are no mounting holes.  I was thinking about using twist ties because cable ties are too think.  If there was a good silicone based glue that would hold the fan on the underside of the heat sink then that would probably be better.
 
Thoughts?

Improvising is a great way to cool your NB. You can use twist ties or whatever, but if you're using glue, make sure it doesn't melt.

Look carefully at the heatsink, you might be able to use screws if the fins are close enough together.  I have added old 40mm fans to heatsinks this way by simply screwing them into the heatsink.  (done this with old TNT2 cards that were passively cooled also.)
 
A word of warning.  I did this to my old S754 K8V-E board.  I took my AIW 9600pro out of the AGP slot, and carefully screwed the fan into place.  I then took my video card and pushed it into the slot.  As soon as it was in and I heard that noise, I knew that I had damaged something.  Taking the card back out, I saw that I had snapped a capacitor off of the video card.  I had failed to take into account that the added height of the fan would have.  If your NB is ANYWHERE near your cards, make sure that the added height of the fan won't interfere with anything.

Agreed with above posters.

See:

Note the NB Fan. The glue used was Silicone so it won't melt until about 80C for a few hrs (~1.2hrs)

RAM Heatsinks on graphics card.

Note MOSFET Heatsinks and laped CPU.

Your work shadow?
 
I don't understand the northbridge fan at all.  Unless the side you glued it to has holes, most of the air its moving will hit a copper (?) wall.  It seems to me that it would do a better job cooling if it was pushing air across the fins, so it should have been facing either the CPU or graphics card.  (no disrespect intended, just wondering why "you" (if you did do this) chose to put it there and not the other way...)
 
To the OP, look at the pics that shadow provided.  Check to see if your heatsink has fins that close together.  If they do, you should be able to find some screws to fit in there.

Don't use a fan. The heatsink is large and good as is. X48 runs so cool, there is no need. If you mess up installation, there may be problems. You have nothing to gain and a lot to loose. Remember, x48 runs at 1600mhz natively, 1680 is only a tiny 4x20mhz oc.

 
Old pic, I noticed this after I took the pic (a few month's back), the fan has since been changed so that now it  sit on top of HSF. I had the image uploaded on  Flicker so just grabbed it as an example about mounting options (ie. use of silicone). Too lazy to grab another pic.  

The Gigabyte X48-DQ6, X48-DS4 and X48T-DQ6 all use a heatpipe system connecting the northbridge to the MOSFET moduals. The northbridge heat sink part of the system is just pinned to the motherboard. But if you remove this system your going to need to add some form of cooling to the MOSFET moduals. On top of that both the DQ6 boards also connect the southbridge heatsink to the northbridge and MOSFET's with a heat pipe as well. You would also have to replace the southbridge heat sink as well.

Exactly, it's possible to even raise temperatures by adding fan.

X48 runs cool. It's no 680i. Just leave it as is.

Most time as said above you can just screw it in. I also did the same thing with old TNT and Geforce cards

old picture but it shows it. If you look at the video card you will also see its an old geforce and there are screw marks in the heatsink

 
Chipsets always run warm dont worry about it, and besides, i had a 965 running @ FSB1600 its whole life with the stock asus cooler (P5B deluxe wifi)

I hadn't planned on removing any copper and I'd like to take it to increase things to 500 but I'd be happy with stable 450.  I figured since I had the mini kaze which is the right size for the heat sink and there's a header for a NB fan that maybe I'd give it a shot.

well in your case you would have to mount in on the side, but to be honest that cooler is more then enough.
 
In my picture it kind of the same. just had a fan laying around. the chipset was never hot(but it was also a pos that never overclocked from day 1 and i tried different cpus with it so it was the board, but its hard to rma over poor overclocking) to begin with.

SirMoby the 945 gigabyte at work I just put a 40 mm fan on it with screws.the p965 ds3 I put this one on : http://www.newegg.com/Product/Prod [...] 6835116018
 
the new gigabyte p35 ds3l I will have on 5-13-08 will probably get this :
http://www.acousticpc.com/coolink_ [...] t_fan.html
hope this helps you.