Hello I'm going to attach a heatsink to my  vert very old ATI Rage 128 (16VRAM) graphiccard. I found a reather small heatsink that will to the job perfectly. I'm going to use ordinary thermal compound but I wonder how will the heatsink hang-on to the card. If I got it right the thermal compound gets stiff after several hours or days of use. But until that day it gets stiff will the heatsink hang-on to the card? The card is located in an old B/W G3 so it's going to be hanging upside-down. I'm afraid that the heatsink will fall off since it's just attach with thermal-compound. Should I be afraid?

yes.  use some mylar tape.

the thermal compound is just for the transfer of heat.

The way to go is thermal expoxy.  Pick up at http://www.frozencpu.com.

Heres a gettho trick for this. If the card has two holes on each corner of the GPU you can use a big paper clip and straightin it out and use it to hold the heatsink on and use two Pencil erasers to hold the paper clip.