Shin-etsu thermal pads (
Link) are the best choice, but it can be a bit tricky to get them to adhere properly. Here is what I recommend:
Clean off the existing thermal tape by soaking the ramsink in rubbing alcohol for a few hours/overnight (or use acetone for faster removal). Lighter fluid may also work, but be sure to degrease with alcohol afterwards. Then, clean/degrease the RAM chip that you want to attach the sink back on to.
Stick the ramsink onto the thermal pad, and carefully peel off the other side. The pad adheres best by application of heat + pressure, so try to heat up the ramsink to 50C or so (a heat gun with a small nozzle attachment is best) for maximal stickiness.
If you have the card in a standard ATX mobo, you should try to apply heat before you put the card back in, as gravity will work against you (I am assuming the ramsink will point towards the ground when the card is inserted). If you have a reverse ATX setup, you can simply run a 3D app to get the card hot.
If applied properly, the shin-etsu pads hold better than the 3M thermal tape I have used, and also transfer heat much more efficiently. Not that it really matters for RAM either way, but it is always nice to know
I have no experience with the adhesive, but I assume that it would be a more permanent solution.
I may have some extra Shin-etsu lying around, and almost certainly have spare 3M thermal tape. PM me and I can send some your way (assuming I can dig either/both up).