I took off the retail heatsink, popped the 2 fasteners from the back of the Voodoo 3. You can use a pair of pliers, or if you have strong nails you may just push the fasteners together that way to get them out.
To mount the heatsink onto the chip itself, we used some 9885 thermal tape from 3M. This is .005" thick, filled acrylic ceramic based double sided adhesive tape so it holds rather well. Most retailers have 6 inches of this stuff for about $3.00, good stuff.
I also decided to add a small fan to the heatsink itself also (I have so many around, might as well slap one on).
I used some industrial strength adhesive to attach the fan to the heatsink. Took about 3 minutes to dry, I also added some more around the edges just to make sure:
I then un-wrapped some 3M tape and strapped the heatsink onto the Voodoo3: