Rth is not usually quoted for any CPU coolers of any kind (unfortunately), so you have to work it out from the data given in reviews.
I have written a program that can do the above calculations, including calculations of Rth. It doesn't support using Peltiers in series, though as I don't expect anyone to actually use such a configuration. Get it here:
http://www.cheese83.freeserve.co.uk/comp/software.htm
[ 12-06-2001: Message edited by: Mr Evil ]