Clean them off, apply AS5 to the center, and put a pinhead sized drop of super glue on each corner of the chip, and then press it on firmly until it sets.
I've done it many times and it has held very well, plus it can come off without destroying anything. You could use some of the arctic silver stuff that is also an adhesive, but once it's on there, it's never comming off.
And there's the thermal tape, but I don't think it would conduct heat as well a a thin layer of AS5.
Here's a 9600XT with pieces of pentium heatsink attached that way.
