They should be placed on a heat sink, http://www.industrialhobbies.com/main.htm send one that is all taped out.
I have 3 320's getting ready to mount them in my enclosure. Do the Gecko's really need to be mounted on a big heat sink? Will they burn up if not? Lets say a continuous draw of 5 amps per drive.... will they burn up if not on a heat sink? If I really need a heat sink how big does it need to be?
I'm not sure if they must be mounted on a heat sink but it's cheap insurance. I got my from the same place that Paul listed about. It worked out great!!