Brad Abrams and Jason Sutherland have come up with a nice little site on Channel 9 which keeps track of compatible versions of CTP's.  The utility uses your browser caps to see what you already have installed, and publishes an XML file that shows valid combinations.  Check it out!

The other suggestion I gave Brad/Jason on this which I'm hoping the community will build is a "Make It So" option.  Basically I want a tool that takes (a) my current machine state, and (b) a new CTP I really want to target, and generates an uninstall/download/install script for me that leaves my machine up and running with all the right stuff.  Should be pretty easy.  Say a CLR Cup<T> mug to the first person to post the sample?  I'll send you two if you write it using IronPython  <g>

[edit: fixed URL to Channel9 site]