Personally I would use a dab of good thermal compound in the center, then a small amount of thermal glue in two (or more) of the corners. Add a small weight (like a paperback) on the heatsink while the glue dries.
A friend of mine did just that to a gforce 2 card... it died very fast...
the glue used expands when hardening. (it was arctic silver, the best)
It is recommended using
only thermal glue.
If the heat sink is to be removed... make a clip...
