What you have now is Napa. Calistoga is the future 800 fsb chipset. During 2007 they will bump Merom's bus speed from 667 up to 800 and offer a new chipset to work with it.
What people aren't quite thinking on here is that because Merom and Conroe are both based on the same chip you are talking about optimizations for these cpu's working on both platforms. That will come on strong as we move into 2007 and new updates to software are written, for example to use under the new Vista OS.
It's the upgrade path for me that finally pushed to get the Core Duo. It'll be a good machine.