For GPU's i am big on using the credit card method.
After the gpu core and heatsink is cleaned with 99% isopropyl alcohol(or as high as you can get)
You just apply a small tab and smooth it flat with the edge(and edge that has been cleaned with the same alcohol) of a credit card.
Now since every video card heatsink is a bit different, the thickness will depend on how close the heatsink fits(For instance my 9800Pro back in the day needed a rather thick layer.). You want as little as you can use while still filling the gap.
Not sure how much, start thin, then apply the heatsink and screw it down, take it apart and see how it looks. This is a pain because it requires cleanings/re application before each check, but it does work to find just the right amount.
@ amuffin, Nice quote and sig