Gonzo,
I don't believe that the ECM of the PT Cruiser has a computer chip that is replaceable like many cars. However, the computer can be reprogrammed for optimum performance. One of the companies that does this is called Kenny Bell. For around $300 you send in your ECM and they either reprogram it or send you one that's already reprogrammed. The only drawback to this is that if you ever take it to the dealer, they could flash your computer while looking for a problem and wipe out all the work that's been done by Bell.
Here's the link:
http://www.kennebell.net/accessories...optimizer2.htm